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?
If you only want to buy items from the game company, the game company just creates an address on their server, and show you the QR code with the amount so the player can pay it. The server is checking if the coins were confirmed by the network and then would give you the items.
It is not necessary to implement any type of wallet inside the game itself.
If you also want that players them self are able to sell and buy in-game items with each other, I would add something like a SPV client into the game where each player control his own private keys. The game would create transactions automatically after the player confirmed the payment and then would exchange the item after the transaction got confirmed.
Instead of a SPV client you could also integrate a lightning client, which would allow cheaper micro transactions (what is usually the case in games).