Am I missing something? I've never had a code for PlayStaion game? Are we taking about trading digital tie in games?

You can buy digital codes of games. you can redeem that code in the store and then download the game.

Dunno how they do it though but cant be that hard

I don't know if you can get game codes but I know I've used codes before for things like dlc and on the website there is a double redemption system. First it tells you the code is active and what it is for, then you have to actually redeem it. I assume if you abort after stage 1 before stage 2 someone else can use it.