Chukwubuikem So 2 things.
this is a matter of how you design your software and let people make in-app/game purchase. So this will require you to partner up with a certified payment provider like Stripe. I also recently came across Tibex, might be worth looking into. Other than this, chances are that the store/company have their own in-game purchase systems. So that might be worth investigating.
Then you have the policy you need to look after. Basically, no store likes you to direct users to an alternative income because that will make them miss out on their own cut. So you'll have to read the rules of the store you plan to release on.
Sometimes there's an SDK that makes it easier to implement a 3rd party service. Alternatively, they have an API you can use. So you'll need to spend some time reading the documents. There's no way around this. Everyone does this.
ArDanZ11 GDScript is probably not powerful enough for that, it would need to be Cpp or CS
Not really relevant. If you can implement any logic/code, you can implement this. It's not a matter of whether it's powerful or not. This is mostly done through making API calls, you are not the one processing the payments, the payment provider is. Then after a purchase, it's up to you to design your system for unlockables etc. There's probably 100 different ways to do this. A common way is to either have a 3rd party service handle this for you (tracks players progress etc), create your own backend system with registered users to track unlocks (your own hosted backend systems and database), or keep the unlocks directly in the client (but then it becomes a bigger subject to cheating).
An important thing that's crucial to keep in mind is to make sure your builds are at least encrypted for security reasons.
