There are several ways to increase population, but in the early stages residences are the most straightforward way to do this. Once you get as far as unlocking the Golden Abyss ancient wonder (if you haven't already) this will give you an amount of population which increases with each level to the point that it becomes very valuable. Further down the line there are also other AWs that add to population. Buildings can also be won in events that give population whilst at the same time increasing your culture level, rather than costing culture.
You cannot in the early game, you need to make a choice which population using buildings you really need.
Each square can only be used once, and making a choice how to populate them is part of the challenge.