Those are all steel frames where those things are usually attached with something like silver solder (or another brazing material), which just flows into wherever the braze-on is touching (or actually into any small gap less than some fraction of a millimetre). Very nice and neat. But it isn't a weld at all. You put flux everywhere, heat the whole thing up, and just touch it with the rod. The braze metal melts and flows into the joint.

OP's frame is aluminium where usually everything is just actually welded. It would be perfectly strong enough to have left gaps at the sides, but it looks a bit sloppy.