Blender Hard-surface

I can’t quiet decipher what Ponti Ryuurui did in the video, it seems like the Ponti joined the meshes together via union boolean, but then after he bevel on top of the cube that was attached to the cylinder, I don’t even know what am saying again but all am saying is that it shouldn’t be possible, at least without the meshes terribly intercepting and causing a heck of a problem, does someone know what trick he used. The addons he has installed for modeling is the meshmachine and the Hardops coupled with boxcutter addon. Thanks.