Affiliate Marketing Secrets

Hi guys. I’m really interested in this crowded niche but what I don’t get is how reviewers (especially youtube) allow game footage. Is it ok to show game footage for reviewing purposes?
Also, how do websites like roosterteeth.com or screwattack.com work? Do they go ” oh, hello EA games, didn’t see you there. we have this gaming review website so send us free stuff and we’ll review it on our website.” How do they get so big they can afford to hire people? How do they make money? Whenever i see videos from they’re HQ’s they’re chockfull of consoles, powerful PC’s, walls decorated with MILLIONS of games (lol). So yea, all that costs lots of money.
I need a reaaaally informed person on the matter. Thank you very much. Hope i asked all the basics