A facebook employee explained me how tracking works. Its not the email address Meta is concerned about. Its the IP, device identifiers and location. Meta doesnt care about the email at all apart from sending you emails for notification. Even with a fake email they exactly know who you are. Let's say you visit CNN.com which has facebook tracker. Facebook has the IP and the device identifiers. Now you login with fake email account on Instagram, facebook knows that's the IP ans the same device hence it "must" be the same person That's how facebook creates shadow profiles.
Meta applies a myriad of advanced and complicated tracking methods. Email is a very popular and easy one. I believe the one you're referring to is called a tracking pixel.
For example, some browsers block tracking Pixels, but if you're logged into Amazon with an email address that Meta knows, they will sell your shopping habits to Meta to show you ads.