They are targeting users who were at a product page (simply looking) which means they are interested for sure. They can't always know whether the users purchase

They are targeting users who were at a product page (simply looking) which means they are interested for sure. They can't always know whether the users purchased it. On Facebook, you can let the system know that you have purchased it by clicking Hide Ad => Already Purchased. If companies use Facebook pixels to track orders, Facebook does know whether you purchased it... but maybe not all companies use Facebook pixels for order tracking so Facebook doesn't know. Regarding YouTube, YouTube ads are simply not worth it. Get YouTube premium. You can ofc disable "personalized ads" in these platforms if you don't want these ads to follow you.