How does one implement authentication using session ticket like in Steam or PlayFab or GameSparks or HeroicLabs?

Question

What is a session ticket used in backend service like Steam and PlayFab? Is it like session_id or is it like OAuth2 access token? Is it the same as authentication token in GameSparks or session token in Heroic Labs? Is there a standard for implementing a session ticket? Why does the majority of game backends not use OAuth2? If a game developer wants to create their own backend, how do they implement authentication with session tickets?