The "Build" mode can be accessed via the main 'Y' menu on the left. Different locations will offer different choices for building.
You cannot build at camps
At FOBs you can build a:
You cannot build at towns controlled by NATO. However, in towns under a state of Anarchy or controlled by the resistance you can build a:
You cannot build at objectives controlled by NATO. If under resistance control you may build a:
Smaller items can also be placed at most locations (see: Placing items)