So in mobile phone Game (Android, iOS), PC game, how to implement that (title)?
In game, game company's specific crypto currency's address, QR code appears for payment receiving,
gamers see that, and send amount to that address, it should be automatically recognize and when it confirmed, game should automatically give item to users.
How this be done technically behind the scene?
I guess you looking for a technical explanation?
I have been working in Payment Gateway solutions, like any other credit card payment there are number of Payment gateway solution offering crypto currency payment too they give you APIs and you simply initiate every time payment is need you get QR code or payment address where client is suppose to make payment. At same time you get event or you inquire using same address for payment confirmation.
In the backend payment gateway company handles the address generation and payment receiving confirmation. They will charge you some percentage of course.
But if want to create you own payment system let say for Bitcoin, then it will be more complicated and you really need to understand more technical things and setup your own bitcoind server and use JSON-RPC commands using the below API
https://en.bitcoin.it/wiki/Original_Bitcoin_client/API_calls_list and this is for JSON RPC, there are example for almost all famous programming languages
https://en.bitcoin.it/wiki/API_reference_(JSON-RPC)
Easier said than done! other then technical expertise there are many things you should be worried about and security might be the biggest concern you have to consider. So it's always better to use existing payment providers to minimize the risks at some cost.