You'll find this setting in the Gmail app for Android under the Profile picture → Add another account. Once a non-Google email account is added, you simply switch between mailboxes with a few taps.

Gmail will now automatically show you a summary card for lengthy email threads if you check a Google Workspace account on the iOS or the Android app.

Google is syncing your web email signature to the Gmail app on Android and iOS, eliminating the need for manual setup.