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.
I’m pretty sure React/React Native doesn’t have any Facebook tracking built in. The dev community would crucify them for that.
I would highly doubt that. They don’t care, most of them not just willingly but intentionally insert google tracking code and similar.