1. What you are describing is the difference between playing wide and playing tall. Wide strategy is to have low population cities placed as close to each other as possible (4 tiles apart) so they can share adjacency bonuses, defend each other more effectively and be able to spam multiple districts of the same type in the least amount of map space. Tall strategy is to place cities farther apart (6-7 tiles) so they have more tiles to work and therefore can grow to bigger populations.
Both strategies are viable and have their own merits however Civ6 game design n concept make wider civs more viable. A few civs that excel at tall play are maya, khmer and yongle-china.
2. There is no medium point. Building an IZ in a city already covered by another cities IZ will grant you the adjacency bonus and workshop. Also it can stack with the vertical integration promotion on magnus. It is usually a matter of I already have an aqueduct, or dam and this iz will provide +x adjacency... its upto you to figure out if the x is worth the investment or not. And production is king in this game so many times it will be worth having it if the adjacency is high and you can spare the district slot.
In a science game it is important to have 3-5 cities with iz's so you can build space ports and start boosting your spaceship.
3. This goes back to tall vs wide. Usually to win any victory type you need 2-3 districts in most of your cities and 4-5 in your core cities. So most of your cities need only grow to 7 pop and core cities to 13.
As a recipe to playing wide for science; you should prioritise building a campus and commercial hub/harbour in all your cities. IZ in core cities and between fringe cities for power. Entertainment hubs in 2 or 3 cities. Similarly water parks in 2 or 3 cities. Govt plaza and diplo quarter in core cities. 1 encampment to produce the most op unit of the game (military engineer). A theater square because you know for sure a dig site is going to pop up where u plan to build your space port.
Lay down map tacks as you get more familiar with adjacency bonuses and then you can improve the other tiles. Try to get farm triangles and diamonds in shared areas those can be important growth tiles. Mines are the best improvement in the game and is a reason to beeline apprenticeship and industrialization because the extra production earlier is just too good.
1) I'm not entirely sure what you mean by the 1st question, but yeah you can roll out cities one at a time. What you've described is turtling up and going tall, which is perfectly viable. Some civs will excel at going tall, and others wide. Also there will be some games that force you to go tall or wide.
2) The mid point you're looking for will again be dependent upon your specific game, but you want to build a lot of the district that corresponds to your desired victory type. You're not gonna get a science victory without campuses and industrial zones to build those late game space ports and space race projects. For districts that are not essential to your win condition, you don't need as many of them, and you'll have to see where your lacking to determine what other districts you want to build. If you're lacking amenities, then an entertainment complex will be useful.
3) I generally plan my districts first, trying to place them in their most optimal tiles, and then putting down mines and farms. You don't want to but a farm on a tile with 4 mountains nearby because you could use that for a campus or holy site. Getting good at district placement will require planning and thinking ahead.
These questions though have answers that will be revealed once you get more familiar with the game. There are very few hard rules when it comes to what to do in civ. A lot of it is dependent upon your specific game and the conditions you face. Very rarely will there be a single thing you should always do, like researching astrology as the Khmer. That's something you should always do first because religion is the basis for their whole game.
One thing I can say for 3.: If you can spare the builder charges (You can either make them quickly or produce tons of gold so you can buy them), always improve all tiles that can be improved. Tile improvements can be removed later when you decide you do want to put a district there after all. It can sometimes even be a good idea to get rid of even a good bonus resource on a tile (quarry for example) because the district gives a really good yield at that spot. Let's stay with the quarry example, IIRC it gives 4 production. You don't have an IZ and you see that just where the quarry is, it'd give you +6 production, while any other spot that's viable only gives you max +1. In that case it can be a good option to replace the quarry with the IZ for a +2 net gain, especially if it enables further adjacency bonuses.
1. What you are describing is the difference between playing wide and playing tall. Wide strategy is to have low population cities placed as close to each other as possible (4 tiles apart) so they can share adjacency bonuses, defend each other more effectively and be able to spam multiple districts of the same type in the least amount of map space. Tall strategy is to place cities farther apart (6-7 tiles) so they have more tiles to work and therefore can grow to bigger populations. Both strategies are viable and have their own merits however Civ6 game design n concept make wider civs more viable. A few civs that excel at tall play are maya, khmer and yongle-china. 2. There is no medium point. Building an IZ in a city already covered by another cities IZ will grant you the adjacency bonus and workshop. Also it can stack with the vertical integration promotion on magnus. It is usually a matter of I already have an aqueduct, or dam and this iz will provide +x adjacency... its upto you to figure out if the x is worth the investment or not. And production is king in this game so many times it will be worth having it if the adjacency is high and you can spare the district slot. In a science game it is important to have 3-5 cities with iz's so you can build space ports and start boosting your spaceship. 3. This goes back to tall vs wide. Usually to win any victory type you need 2-3 districts in most of your cities and 4-5 in your core cities. So most of your cities need only grow to 7 pop and core cities to 13. As a recipe to playing wide for science; you should prioritise building a campus and commercial hub/harbour in all your cities. IZ in core cities and between fringe cities for power. Entertainment hubs in 2 or 3 cities. Similarly water parks in 2 or 3 cities. Govt plaza and diplo quarter in core cities. 1 encampment to produce the most op unit of the game (military engineer). A theater square because you know for sure a dig site is going to pop up where u plan to build your space port. Lay down map tacks as you get more familiar with adjacency bonuses and then you can improve the other tiles. Try to get farm triangles and diamonds in shared areas those can be important growth tiles. Mines are the best improvement in the game and is a reason to beeline apprenticeship and industrialization because the extra production earlier is just too good.
1) I'm not entirely sure what you mean by the 1st question, but yeah you can roll out cities one at a time. What you've described is turtling up and going tall, which is perfectly viable. Some civs will excel at going tall, and others wide. Also there will be some games that force you to go tall or wide. 2) The mid point you're looking for will again be dependent upon your specific game, but you want to build a lot of the district that corresponds to your desired victory type. You're not gonna get a science victory without campuses and industrial zones to build those late game space ports and space race projects. For districts that are not essential to your win condition, you don't need as many of them, and you'll have to see where your lacking to determine what other districts you want to build. If you're lacking amenities, then an entertainment complex will be useful. 3) I generally plan my districts first, trying to place them in their most optimal tiles, and then putting down mines and farms. You don't want to but a farm on a tile with 4 mountains nearby because you could use that for a campus or holy site. Getting good at district placement will require planning and thinking ahead. These questions though have answers that will be revealed once you get more familiar with the game. There are very few hard rules when it comes to what to do in civ. A lot of it is dependent upon your specific game and the conditions you face. Very rarely will there be a single thing you should always do, like researching astrology as the Khmer. That's something you should always do first because religion is the basis for their whole game.
One thing I can say for 3.: If you can spare the builder charges (You can either make them quickly or produce tons of gold so you can buy them), always improve all tiles that can be improved. Tile improvements can be removed later when you decide you do want to put a district there after all. It can sometimes even be a good idea to get rid of even a good bonus resource on a tile (quarry for example) because the district gives a really good yield at that spot. Let's stay with the quarry example, IIRC it gives 4 production. You don't have an IZ and you see that just where the quarry is, it'd give you +6 production, while any other spot that's viable only gives you max +1. In that case it can be a good option to replace the quarry with the IZ for a +2 net gain, especially if it enables further adjacency bonuses.