How to allow user to create and switch between multiple accounts in Flutter?

In the default scenario, Firebase Auth generally does not support allowing a user to be signed in with multiple accounts at the same time. If you want to add support, what you will have to do is use initailizeApp() to initialize a new App instance - one for each user account, and sign in the user to each one of them. You will then have to pass that app instance around to the other Firebase APIs to use that account for authenticated access (for example, Firestore queries).

To be honest, it's not clear to me from the provided APIs how to do that last part. but perhaps Firestore.getInstance(app) might do it.

In any event, it is not trivial to implement. There is not a simple configuration or trick that will allow multiple simultaneous sign-ins. Usually apps just make the user sign out, then in again with another account.