>I expect they pay a flat fee to license content

I wouldn't tbh, though I'll admit I'm speculating solely on public information. During the 2023 strikes, SAG-AFTRA and the WGA negotiated additional residuals based upon whether 20% of the streaming services subscriber base viewed the content within 90 days of release.[1] So, streaming platforms are evidently willing to share subscriber viewership data with 3rd parties if it's a contractual requirement.

I would be surprised if content licensors haven't negotiated an as good or better deal for themselves.

[1] https://variety.com/2023/biz/news/sag-aftra-streaming-bonus-...