The first video shows that one of the receivers was assembled using screws and silver solder, for those who can't weld. The screws were cut flush with a grinder.
The second video sheds a bit more detail on the construction. The buffer tower was assembled via wrapped sheet metal around a buffer tube castle nut and presumably soldered or welded in place. Seems a bit more complex and time-consuming than my proposed steel lower ideas because I think hammer forming sheet metal takes a lot of trial and error and guesswork - however, I do think that this thin sheet metal method does look more refined and less 'chaotic' than my Home Depot lower.
I'd like to try something like this eventually, and when I do I will report my findings.