FB knows your eBay, for that matter. When I search for something on eBay I get ads on FB for it.

Even if they are not merged, corporations can share your data behind the scenes and you are forced to unilaterally, without negotiation, accept that ToS to use these sites.

It seems safe to assume that just about any vaguely commercial site knows about your activity on all the other ones. Sometimes the cookie banner shows info about who they share your data with. There will be literally hundreds of entities. And I’m sure many of them pass the data on further.