How do the “legit” popups on piracy sites work?

when I watch sports via the streaming sites? A lot of the ads are obvious scam popups or redirects to someone’s channel. But then a lot of times it’s an ad for betmgm or other legit businesses.

How does that even work? Are they fake links or domains? Or is there some weird way/reason that they’d (the streaming sites) get paid for redirects and clicks onto say BetMGM. Cause I highly doubt it’s some direct deal like a TV deal