Isn’t that all to sell you ads?

For the general "you" yes, but not the specific "you". It would be possible to use your data to sell someone else ads.

But they don't? They use data internally to improve ad revenue but I doubt they ever sold data for money or intend to do so.

Do you know what the legal status is of models derived from someone else's data? EG, can I train an AI from every Marvel movie, then use that to produce other movies?

That's not a great analogy because I would be violating other people's IP, whereas FB generally has rights to the data they collect.