>> I am genuinely curious who can actually threaten Visa (I do not think it is Valve).

> Likely Apple currently has the deepest finance industry roots.

Apple used a very large bank headquartered in the US for its credit card processing as of about ten years ago. Given that the cost of change is significant once these processes are put in place, it is likely this remains the case.

Note that this is not the same as what Apple Pay supports.