Hi, I was wondering how much functions can be built into a custom wallet? Like can I code a game into a wallet? Or can use API to add in features to a wallet?
As this question is very generic, I'll answer it in the same way.
If you wanted to code a wallet from scratch, you could code any kind of functionality that you desire into a wallet, as long as you can code it yourself or integrate it from existing code.
You can also extend a wallet of which the code is open source. You basically create a fork of the code and add the functionality you need.
However, the easier and more widespread way of creating software that is based on bitcoin is to use a wallet with minimum functionality like bitcoin core and then you can code a standalone application in whatever code language you want to, and then your wallet software communicates with the running bitcoin core via REST API requests.
Without knowing what your goal is, it is hard to be more specific here.