Finite Resources
As in the real world, resources are limited. However, you do have the ability to reclaim/recycle materials you are no longer using.

Entropically Stable
The game is entropically stable. Leaving a singleplayer game running overnight will not result in citizens dying and the cities falling apart. Only players can increase entropy within the game.

Food
All units in the game are required to eat food at regular intervals. Citizens need to be in close range to food storage buildings in order to be fed. Food sources are infinite, however the production rate is finite and can require cooldowns.

Needs Resources
You are able to task your people to create structures such as buildings or farms whenever and as much as you like, but they will complain if you do not have enough resources for them to complete their projects.