BoxSession and login flow

Hello,
I am using the SDK for Android.
I want to use my own login flow with a webview through the OAUTH2 rest API.

Everything is ok, I get a token and a refresh token, but I don't know how to build my Boxsession from these tokens and the userId.
Knowing that I must be able to switch between several accounts ...

The documentation of this SDK is very imprecise on this subject.

A track would be appreciated.
Thank you in advance.