You've set up multiple email addresses but just realized the wrong one is your default. Here's how to change your iPhone's ...
To change the default email app on your iPad, you'll need to be running iPadOS 14 or higher, and head to your Settings app.
You can add multiple Google accounts on your Android device. However, it considers the first account you sign in with as the default and uses it to sign you in across various Google apps and services.