I am a bit of a min/max-er and the district placement puzzle is one of the aspects of the game that I enjoy more.
It feels reasonably easy at first glance, but when wonders, unique districts, and multiple eras come into play it can become a pleasant head-scratcher.
If you have a lot of space, then you just go for the best adjacency you can get, but this is often not the case.
Here is how I go about it:
My first consideration is 'creeping'. Since districts have to touch one another, you need to path out to where you want to place other districts.
For coastal cities that means you have to get across the water, with water districts, to get to small islands, or around resource hexes that sometimes 'block' you in.
I wish wonders and resource buildings would count as districts, for the sake of city extension, but I have accepted it as a part of the fundamental puzzle, as is.
Water:
My Fishing quay, Lighthouse, Shipyard, Wharf and Port 'bridge out' towards other land squares or resources that I want to reach, while still having the best adjacency, and leaving room for water wonders that I intend to have.
I tend not to stack them, since there are normally enough water hexes to have them all without overbuilding. But stacking them is fine if that gives a better bonus, or there is limited space / no need to connect.
Food / Growth:
1) I try and pair my granary, ultimately with the grocer in the modern era, away from resources, wonders, mountains, coast and lake, since other districts benefit more from those. In the interim I may add another 'aged' building to it that I am OK with overbuilding eventually.
2) Bath + Garden ==> Hospital + Inn ==> Cannery / Tenement
The bath does not have coastal adjacency like the others (, perhaps they forgot,) but I place it, as if that it has.
Culture:
1) I tend to pair the monument with the amphitheater, then kiln / pavilion and finally museum / opera house, next to a mountain and/or wonders. Overbuilding, unless I have the luxury of space.
Science:
1) Same thing for the library / academy ==> observatory/university ==> schoolhouse / laboratory, but adjacent to resources.
Production / Military:
I try to reserve 4 spots for these, adding over the ages, and prioritize them higher.
1) Sawpit + Brickyard.
2) Sawmill + Gristmill. I prefer a nav river square for these, since not many other things can go there, otherwise a normal river with 'lowest' adjacency. I know Gristmill is not a production building, but the 2 mills go together by design.
3) Barracks + Blacksmith ==> Dungeon + Armory ==> Military Academy + Factory
4) Stonecutter + Ironworks in the next era.
Happiness / Religion:
1) Arena + Villa ==> ... ==> Park + Department Store
2) Altar ==> Temple + Menagerie ==> Radio Station
With the exact Altar adjacency bonus determined by you selected pantheon.
Commerce:
1) Bazaar + Guildhall.
2) Market ==> Bank ==> Stock Exchange, in the city center, unless there's space.
Transit:
1) Rail Station in a low adjacency spot, where I can manage, for the factory unlock.
2) I tend to only build 2 Aeros and one L-pad in a high prod city, if I'm going for the Science victory.
I wish the port / aerodrome would 'connect to the capital' without a rail station for island / isolated cities factory unlock.
Bridges:
As needed for extending, without taking spots from more important items.
Unique Quarters / Wonders:
As the situation demands.
You can intermingle science with production, culture with happiness, commerce with food, and warehouse buildings, since they tend to have the same adjacency bonusses.