It’s net movement, not gross. There’s raw data that shows all 53 x 53 of how those born in the 50 states, District of Columbia, Puerto Rico, and overseas live in the 50 states + DC + PR + non-US. I can’t do the analysis easily from my phone, but you can break it out to what percentage of Americans live in a different state of birth. There’s probably raw data that can show the Gen Z specific stats, too.

You might be right. I don’t know whether those American operations are the primary manufacturing bases for those European companies’ non-American customers though.

most buy those firearms and other weaponry from the USA

Do they? European nations have pretty well regarded small arms manufacturers, who even American law enforcement agencies tend to buy from: H&K, Glock, SIG Sauer, Walther, Beretta, Benelli, CZ, etc.