You "Login with Apple" or "Login with Google". They manage the login entirely and pass me your id and an access token (assuming you pass their login test). I store that in my DB so that your data from the app can sync (the paid-for app syncs your training data to my backend but I match it only based on the Google/Apple id.)

The alternative is that I build my own auth system and I'd need to store something you can type in the next time, e.g. email/password address etc.

If you have an Android/Apple phone you're already authenticated with them. I just need Google/Apple to say "this guy is cool, let him in" and I then use the id to check if you've paid, sync your training data etc.

On its own, the id is useless! Means nothing and cannot be traced back to a person. I genuinely do not know your name, email, what country you come from, GPS data, CC data. Nothing at all!

I don't want your data.