"Sign in with Apple" does have a very bare-bones OIDC flow in the browser on non-Apple devices/browsers. That flow takes you through a typical Apple Cloud login

"Sign in with Apple" does have a very bare-bones OIDC flow in the browser on non-Apple devices/browsers. That flow takes you through a typical Apple Cloud login flow with Apple ID email and password and usual 2FA if it's the first time on that browser/device. To meet bare minimum requirements sites like Calm.com often only show the "Sign In with Apple" button for iOS and iPadOS user agents. You can sometimes fake an iOS/iPadOS user agent and get the buttons to show up and then switch the User Agent back to get the boring in-browser OIDC flow. As an iOS/iPadOS but Windows PC user I find it frustrating how many websites intentionally hide that button in other browsers, just when dealing with my own accounts. I feel like Apple could maybe do a better job of education here: "please don't hide the button on non-iOS/iPadOS user agents". I hope Apple has strong developer education plans for Passkey.