“Is it completely legal to manufacture a firearm without a federal license?”
Yes, you just can’t sell it. You can get all the parts to make AK’s AR’s 1911’s at home if you are good at using tools.
Those are pretty well made for slam fire shotguns, real fittings, no duct tape in sight.
And for the purpose of a gun “buy back”, the guns are all assumed to be illegal and it’s all done “no questions asked”. They rip off some old ladies who don’t know the value of their husbands old gun, but mostly they get stolen guns, crime guns, and broken guns.