Authorization / Registration of users in the Unity game. Who can suggest a couple of points?

Question

C++ / C#

Hello!
In short: there is a multiplayer game written in Unity. Unity client, c# server, communicate via TCP. For mobile platforms. There are a couple of questions about the process of registration and authorization of users
1) What is the best way to do the authorization / registration of users? It came to mind to make a separate registration server written in php. To send post requests to it from the client, a certain REST API. Receive a token from the server and then use it on the client.
2) How to store user authorization data in the unit. So that each time you enter the application, you do not need to go through authorization again. Do they do it through PlayerPrefs? What to store? id and token?
I would be glad for any useful information on this topic. If something is not written, ask. Thank you!