I don't have any metrics to back this up, but I would assume most websites that use these third-party login systems, still pull down your email address and create an account for you based on that. So it stands to reason, you if you used the same email for all Facebook, Google, Apple, you could sign in with any of them and maintain one account.

I suppose that's a huge assumption, but that's how I would do it if I was developing against them. That said, it doesn't help w/ the "Hide my Email" or the default icloud.com email addresses people don't realize they're using.