How do I get user info in EE3 in an extension?

I’m listening to the hooks “sessions_start” and “cp_member_login”. I want to be able to know on every CP page:

1. The current logged in user
2. What member group they’re in
3. What they have access to

And possibly other info. How do I get this?

I tried using the session object passed by the hook, but the info was all empty.

public function listenToSession($sess)
{
    //This is false
    $userName = $sess->userdata[ "username" ];

    //These are both empty
    $memberId = $sess->sdata[ "member_id" ];
    $sessionId = $sess->sdata[ "session_id" ];
}