Districts define the core functionality of your base. Each district corresponds to a unique function with its own separate bonuses that compliment the overall experience. They also have levels that unlock new features and content. To level up a district you need to gain Upgrade Points for each one separately. Upgrade Points can be gained by Improving your district.
Every district has it's own set of Improvements, and each one has specific bonuses and rewards in terms of Force and Upgrade Points. All improvements have a construction cost in one of more resources and also take a certain amount of time to complete. When starting an improvement you occupy one Build Slot from the Build Queue. There is only one such slots at the beginning of the game but you can open a second slot by using a corresponding item. You can also finish up an Improvement construction for free and the time for that depends on your current active VIP level. Once an Improvement is complete, you gotta confirm the construction manually before the corresponding bonuses are applied to your District
The same construction rules apply also when building your Headquarters or any of the available Add-Ons