If signInWithPopup from Google Firebase Authentication isn’t working on iOS when using Ionic + Capacitor, it's likely due to WebView limitations. iOS does not support pop-ups the same way browsers do, especially in WebViews.

Possible Issues & Fixes:

1. signInWithPopup Doesn't Work on iOS WebViews Issue:

signInWithPopup() relies on pop-ups, which are blocked by WebViews in iOS. iOS does not allow pop-ups from WebViews due to security restrictions. signInWithPopup() works fine on browsers but not in an embedded WebView.