No, they aren't. Many creators are "demonetized", and if you ask the ones who aren't, they'll tell you they barely make anything from YouTube itself. They get all their money from other sources: Patreon, merchandise sales, sponsor segments, etc.
I watched a couple of guides by rich streamers, and they consistently point out that the unofficial means make them the most money.
When they're very big, yes. Mid size and small channels, I doubt it.
Small channels make their money from super chats — a handful of $20 super chats will outweigh your ads until moderately large.
I don’t know about YouTube premium, but regarding twitch partnership they said that only the top streamers get good deals, but even then they are problematic because they require exclusivity.
Yeah, but then you're giving money to Google. Better to pay the creators directly.