Additive and Subtractive Mesh
You are asking for boolean modeling, something that is difficult to do even in full-fledged 3D suites. It may sound esy to the user, but the geometry creation is very far from trivial.
interesting, i figured because this was built in unity i figured it would support this, as the only engine i have used is UE4 and figured it was something that was universally supported. im sorry it seemed so stupid...
Thing is, it is comparatively easy to do in parametric 3D, where objects are mathematical constructs first and foremost and any mesh geometry will be generated wholesale by the software. That's why it is a staple in CAD products. AssetForge does not work with parametric 3D objects, it works with pre-made meshes. Cutting pieces out of them, stitching them together and deciding where faces and vertices have to be placed to not damage the pre-existing geometry but to also modify it in accordance with other geometry that joins with it/cuts it is a whole different kettle of fish.
If I remember the Unreal engine correctly then it is not "universally" supported, it only applies to the level editor, which basically is a fancy volumetric object editor. It's not applicable to the various geometric meshes you import to populate your scenes with or use as actors.