How would one go about creating a fully FOSS or mostly FOSS alternative to Google Pay?

Do not know of any open source alternatives and it is surprising to me there are none. To my understanding, all that is needed as a mobile app that one can store their credit card on (number, expiry, cvc) and have NFC for permission for tap payments. The app should be able to function offline since the payment is done though the NFC, not your phone’s internet connection.

Im not a fan of no identifiers. Sure simplex is secure with it but I would like to see something like simplex that uses 24 word seed phrases to generste millions of unique identifiers the user can easily backup and restore from a piece of paper and not from a digital backup file.