Buildings are your main source of Cash income. A building will start generating Cash once you've unlocked them. Additionally, you can assign capos to manage your buildings. Assigning the right capos will activate a Cash multiplier that will increase your Cash income for that building.
There are 2 ways to unlock a building:
1. Pay Cash. The good ol' bribery, if you want to do things the nice way.
2. Fight for it. Well, if they don't want to listen...
In order to optimize your Cash production, you can turn on automation for your buildings. Each building needs to be managed by a capo with a specific level, to activate its automation.
Please also note that each building requires a certain category of capos, such as BIZ, TECH, or STR. Take over as many buildings as you can and assign your team of high-level capos to manage them for you!