Amazon, Walmart, Target and then increasingly unsure.
Visa is a clearing house whose members are banks. Think of it like a payment router between issuers (banks) and processors (banks).
Only sponsored organizations can directly use the "Visa rails", where "sponsor" is defined as a bank, a bank subsidiary, or an entity previously sponsored by one of the other two.
This is also the case for MasterCard and Discover. "Traditional" American Express is different though.
> Amazon, Walmart, Target and then increasingly unsure.
Those merchants use banks or one of their subsidiaries for processing credit card transactions. Most large merchants do as well in order to minimize their discount rate as well as other transaction fees. Smaller merchants often use ISO's or VAR's for business specific reason, knowing both ultimately transact with a bank or one of a bank's subsidiaries.
I know at least two of the above used to use a specific US bank for the credit card transactions backing their payment services. For others, if service usage requires a verified credit card or debit card backed by a credit card network, they too use a processor owned/operated by a bank, bank subsidiary, or an entity sponsored by same.
EDIT:
For payment services which do not require a credit card or debit card backed by a credit card network, they almost certainly use the ACH[0] network. This is a more intimate financial relationship and best used with a dedicated bank account not linked to any others, as fund transfers can be bidirectional.