My rule of thumb is that most of streamers use fake money or a mix of actual funds and bonuses provided by the casino.
If the streamer favors an specific casino over other, it would be easy to assume there is some deal going on behind the scenes, it is the same with those webpages to gambled with Counter Strike skins and cosmetic objects. I have read about cases where very popular youtubers record themselves gambling on platforms they personally ownership over.