From the NFA handbook:
However, certain alterations to a pistol or revolver, such as the addition of a second vertical handgrip, create a weapon that no longer meets the definition of pistol or revolver. A pistol or revolver modified as described is an “any other weapon” subject to the NFA because the weapon is not designed to be fired when held in one hand.
Gun laws are stupid.
”A woman who demands further gun control legislation is like a chicken who roots for Colonel Sanders.”
~Larry Elder
It is illegal to put a grip on a pistol. It is like applying to the ATF for a class 3 weapon if you do it. Seeing all the kni of weapons he shows on his videos, I have no doubt he has the licenses for it. Also, he probably is very rich too and yes he is not really Russian.
He doesn't have an FFL last I checked, but he does work for one, and is probably how he has access to all of the firearms he does. I'm sure the individual who owns/operates the firearm business is somewhere on premise when he does these videos.
You're welcome.