I'm an iOS user. I logged into PikPak and found that the email address bound to it is unfamiliar

They end in @privaterelay.appleid.com email address is a random email address that Apple automatically generates for you. This is because you chose to use an anonymous email address when you signed in to PikPak with your Apple ID.

As long as you don't turn off this feature, emails sent to this email address are automatically forwarded to your Apple ID's real email address.