How To Verify A User's Id Token In Firestore Using Javascript?

I am building a react native application and am using Firebase, more specifically firestore, in order to manage my data. My current objective is to implement an auto login feature

Solution 1:

I then want to save this token into the phone, so when the user opens the app again, I can fetch this token and check its validity.

This is completely unnecessary. Firebase Auth with persist the signed in user, and automatically refresh the token without you having to do anything. All you need to do is listen to when updates to the token are made available, and act on the new token as needed. You can establish an ID token listener using onIdTokenChanged as shown in the linked API documentation:

firebase.auth().onIdTokenChanged(function(user) {
  if (user) {
    // User is signed in or token was refreshed.
  }
});

Once you have this token, you know that the user is successfully signed in. There is nothing left to do. There is no need to use it to sign in.

Also, you can't verify the token on the frontend. The verifyIdToken method you're looking at is for the Admin SDK only, which only runs on the backend. The idea is that you get the token on the fronend, then pass it to the backend as described in the documentation for the Admin SDK. The backend uses this to securely determine if the user on the frontend is who they say they are.

Since you didn't say if you have a backend or not, dealing with this token might not be necessary at all. If you just want to know when the user is signed in (even if they are just returning to the page after being away, then you can skip everything above and just use an auth state observer. Again, Firebase Auth persists information about the user so you don't have to sign them in again. The observer will tell you when the automatic sign-in is complete, or if they are not signed in at all.