Hi DesuDeus, no problem, I'm glad you were able to solve it already! :)

Yes - Join is for gluing together individual surfaces with currently unattached edges to other adjacent surfaces.

Join only glues edges together and does not attempt to cut, intersect, or discard any geometry.

The boolean commands like boolean union are focused on intersecting objects and combining the cut up pieces together, removing different pieces depending on which volume they were in.

- Michael