At the beginning of you career you have chosen a map where you wanted to build your first depot, after that you start driving tours in OMSI 2 to earn money. The earned money can be invested in new depots on new maps. All maps for OMSI 2 are compatible with the career mode.
Every depot has 5 parking spaces, that means you can buy and use 5 buses per map. If all of those are in use and you need more buses, you can use your money to extend your depot. For every depot extension you will get 5 more parking spaces in the depot.
It is also possible to sell depots, but you will only get a part of the costs back. Selling the depot is only possible if it is empty. If there are still buses in the depot, you first have to sell them.