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?
There are several possibilities to implement it.
You can either use a 3rd party service to process the payments or you do it yourself.
An example: Once a customer decides to buy an article, you request a new address from your local daemon (or take one unused pregenerated address)
and assign it to your customer in your database.
Then you display the QR and after the (correct) transaction has been received by your node and has the amnount of confirmation you decided to set,
you can afterwards release the digital item to your customer.
Im not sure 'how behind the scene' you are asking for.
But there are a lot of code snippets available on github which can deliver you an technical insight.