App Store prompts for Google password despite primary Apple Account

when I need to update an app or download a new app I keep getting a notice to sign in with my Google email password. I have a Google email but it’s not my primary account.

[Re-Titled by Moderator]

Original Title: why does apple keep asking for my Google password when i tryvto use the App Store even though my primary id is an apple email account? Google doesn’t need to know everything I do on my iPhone!