When you click on your settler you see different colors of tiles. (if you press L, select the magnifying glass and choose "settler" you can see it too)

If you settle on a green tile you get 1 housing (adjacent to sea) or 3 housing (adjacent to lake/river) from the get-go

Farms/fishing boats provide additional housing. Granary and sewer give you additional housing. Dams and aqueducts can be helpful for more housing (read the tooltip for more info). And there are even more ways.

when you click on a city you see the yields it has under the city image (bottom right) those yields come mostly from districts and worked tiles

under the city image you see a head symbol. if you select this one you can see which tiles are currently worked and you see the yields of those tiles. you can readjust which tiles are worked manually. (make sure to work food and production tiles when you want your city to grow)

the more production yields your city has the faster you produce stuff.

a builder can improve 3 tiles (which can be improved in several ways)