I know a guy who does fortnite content and he brings in about 100 max at any one time. So to answer your question, you do earn some off twitch itself, through subscribers and donations. But this guy I know also does amazon affiliate links, he has a fornite app that people use to find treasures and runs ads...

He also told me that Fortnite gives him a unique affiliate code. So basically if people buy skins and stuff in game, he gets a cut. So it just depends on your following and doors start opening.