Hi mha94,
Thanks for reaching out in Apple Support Communities. It sounds like you would like to know why you can make in-app purchases using your Apple ID balance for the Genshin Impact game. We're happy to help and to provide you with more information. "Some purchases might require that you have a credit card on file, even if you use your Apple ID balance to make the purchase."
Check out the following support articles:
What you can buy with your Apple ID balance
Buy additional app features with in-app purchases and subscriptions
How apps, content, and subscriptions from Apple are billed
If you have any other questions, reach out for additional support here: Get Support
Cheers!