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They could use inline commercials.
They could also disable viewing while not logged in, they count the minutes you’ve watched content vs the minutes of commercials that content was supposed to have and block your account.
Addblocker that runs a second instance in the background that watches the add for each video to make up the time.
I think there’s an addon for Twitch that does exactly that. It switches to a different feed when Twitch tries to play an ad, then switches back after it’s done.