The inventory can be found in-game while playing a map on your HUD (left upper corner) and can be opened /closed by clicking. It has eight slots that can be filled with crafted modules and four slots for objects. The modules/objects can then be dragged out of the inventory into the in-game area on towers/buildings or free space when it´s an object.
Once a modul is placed on available in-game area, it can not be dragged back to the inventory again. Same behaviour for objects.