Reminds me of a meeting I was party to with the Safari team. We worked with them on some standards stuff at an old job. They claimed to have creepy-level tracking of users back then. We were discussing how to identify users for an A/B test across millions of sites and comparing what fingerprints we could both derive to most likely end up on the same user.

If you use a closed source browser. That’s the kinda shit they do.

Just using IP address, device storage, device name, and similar signals, we can identify a user. It isn’t difficult to correlate these data points. Apps like Facebook also force developers to use their SDKs for even small features.