How does Firebase Analytics define a "user" (for purposes of Audience)?

Question

I've been told that, "Once a user is in an audience, they will forever be in that audience". I'm curious how long is "forever," in the context of a user? For instance, if a user changes phones, or uninstalls the app and then re-installs later, does the user persist across those transitions, or is each of those a separate user? And if I have multiple apps in one Firebase project, does the user appear in the same audience in all apps within that project?

What does Firebase Analytics consider to be a unique/new user?

One use-case I'm looking to try with Audience, is I want an audience of "existing users" (defined by "first_open_time <= {some date}") and "newer users." I have some changes I want to implement for new users, but I want to let existing users have "legacy" behavior for a while, transitioning them more slowly. If a person was in the "existing user" audience, then got a new phone, or uninstalled and later re-installed the app, would they suddenly be a new user, and therefore no longer be in the "existing user" audience?

Another use-case: I have an existing app, and I am writing a whole new app, which will have a new package name, and will therefore be a new install for all my existing users. I want to transition them all to the new app for free, and the best way I can think to do it is have both the old and new app be in the same Firebase project, and put users of the old app in an audience, and when they run the new one, if they are in the "old app users" audience, it will give them the premium features automatically.

To be confident in either use-case, I need to better understand the definition of "a user" for Firebase - does it persist across uninstalls, transition between devices, and can you share audiences between apps in a project?

Thanks!

Answer 1

In Firebase Analytics, a User is synonymous with an instance of an app. So:

1. If your app is installed on Device A, then uninstalled, then re-installed on Device A, you will have two distinct Users.

2. If your app is installed on Device A and then later on Device B, that counts as two distinct Users.

Answer 2

Too late but for somebody.
Follow google support's User-ID and Cross Device. And About the User-ID feature details about benefits of having our own User ID.

Steve Ganems's answer is right. Without user id, user's uniqueness is based on app's instance. Once it is lost(uninstalled), that user is lost. So obviously the app in different device is considered as a distinct user.

From doc:

Without user ID, a search on a phone one day, purchase on a laptop three days later, and request for customer service on a tablet a month after that are counted as three unique users in a standard Analytics implementation, even if all those actions took place while a user was signed in to an account.

When you implement User-ID, you can identify related actions and devices and connect these seemingly independent data points. That same search on a phone, purchase on a laptop, and re-engagement on a tablet that previously looked like three unrelated actions on unrelated devices can now be understood as one user’s interactions with your business.