Building and selling
Fill your base's slots with buildings, one at a time.
Open a base and pick a building from the build menu. You pay when you order, and the base builds one building at a time, in order.
You can cancel an order and get your money back. A building already under way refunds the part not yet built.
Finished buildings can be sold to free their slot. Selling takes a little while, can be called off, and only gives back a small part of the price.