I've tried to select all, remove faces, invert from another post etc... however my faces still overlap. It's one mesh, here's a photo. The overlapping is very apparent where the objects intersect.
Your topology is not good, for example when you cross two parallelepipeds you don't just duplicate one and rotate it 90°, you need to create edge loops, extrude or bridge, etc... so that faces don't overlap. Also try to work with quads.
Note that in some cases it's ok to just intersect meshes (for example if the pieces don't need to appear welded).