Retreive Achievements List with player not signed in

which is the best practice to retrieve and manage achievements in a mobile game when the player is not signed in?
As far as I know achievements are only available when the player is signed in. I read about anonymously sign in with firebase but you have to implement the achievements in firestore.
As far as I understand, you can't use unity anonymous sign in to manage achievements.
Any thoughts?

It really depends on the type of game you are building, your game design and which online achievement solution you are using.

You can manage your achievements locally and persist on disk until you login and then perform a synchronization.

Once logged in, you can use cloud save (and optionally cloud code for security) to build an achievement system that will be cross-platform. You can synchronize these with google play games achievements if you wish to.

My idea is to use Google play and Game Center achievement solution, in order to let the player improve his/her statistics. The drawback of this approach is that you must have two different copies of the achievements list, one locally and one remote and sync them, with conflicts resolution.
Otherwise I have to implement a custom achievements system based on a cloud service independent from Google Play and Game Center.