Hi Lav,
If we talk about the backend of turn based game, it is based on two things. One is saving the game state and another is to notify the user once one user has done his move.
Using App42 these two things can be done using Storage Service and Push notification Service. In Storage Service you can save the players move/game state and using push Notification service you can notify opponent user about the move. Here are the two useful links which might be helpful in getting started with these services.
· Saving Application data
· Push Notification on Android/iOS/Unity
Let me know if it helps or you need any further assistance here.
Thanks
Ajay