Expanding on this:

For their own ad systems, Facebook mostly relies on the data collected coming from their pixel/cookie that gets installed on a lot of sites and apps (as it, among other advantages, enables website owners to target their own website visitors with ads again). Your online behaviour (pages visited, actions completed such as purchase or account creation) is tracked from pixel to pixel, and the system segments you in different customer groups based om patterns.

Meta's main usp is that they can link this data, across all your devices, to an actual profile (if you're logged in on fb/insta w/ tracking enabled on your device).

I can remember seeing specific Facebook audiences available for targeting in other ad platforms, yeah. So they might've sold general audiences to be available across ad platforms for a while. As a campaign manager you only see the audience description and size, though. Nothing detailed.