Google Pay Android Tipping

I've followed this google pay android tutorial: https://developers.google.com/pay/api/android/guides/tutorial#java and I'm confused af. I've got the user credit card data in a tokenized form, great, and now what? Why is there no tutorials online that explain setting up google pay from start to finish?

I want users to be able to pay whatever they like in-app, like a tipping system.

Anyone have any solid resources on this use case?

Thanks a bunch!