If you made a pixiv account with your AppleID, you can log in using the following methods.
■ Login methods
pixiv login page > Click on "Continue with Apple" or the Apple icon
If you can't remember your password, you can enter your registered e-mail address at the URL below to reset your password.
■ Password reset form
https://accounts.pixiv.net/password/reset
If you can't reset your login or password to the e-mail address where you receive e-mails about support and updates, please try resetting your login using the @privaterelay.appleid.com address.
You can check the @privaterelay.appleid.com e-mail address that you used to register using the procedure below.
■ How to check your e-mail address
1. On your Apple device (iPhone, etc.), go to Settings > Apple ID (account name) > Passwords and Security > Sign in with Apple > pixiv
2. Reset the password for the e-mail address ending in @privaterelay.appleid.com displayed under "pixiv".
If you made an account using an Apple ID, your account may not be registered with the e-mail address you used to register your Apple ID, but with the e-mail address ending in the domain @privaterelay.appleid.com.
If you have set your Apple ID e-mail address to private, when you registered on pixiv with your Apple ID, you were registered with a randomly assigned e-mail address ending in "@privaterelay.appleid.com". E-mails sent to your @privaterelay.appleid.com address are automatically forwarded to the e-mail address associated with your AppleID.
To learn more about privacy settings, please check Apple's Help pages.
https://support.apple.com/105078