I feel like you are asking many different questions here all at once. In regards to the base building mechanic you are probably interested in the gridmaps.
Modular vehicles is a bit too vague I think. Could be interpreted and for that matter approached multiple ways. Depends on your projects actual needs. You should probably revisit this at a later time when the rest fo your project has taken more shape.
For the basics of making a inventory system you should be able to google godot inventory system
and find plenty of different tutorials. After watching and maybe following a few you should have a reasonably good idea on how to tackle that.