Our card processor is Fiserv EFT. We file the dispute thru their system (we are a VISA issuer), and Cash App responds. I haven't had to go to pre-arb to get responses from Cash App either which makes me happy. I hate it when the merchants first response to a dispute is just a reiteration of the dispute we filed, and then we are forced to take it to pre-arb to get any real evidence. Cash App gives me the name on the account, phone number, IP information, possibly the email address. It's enough evidence that I have revoked provisional credit before or it's convinced me it is truly fraud.
We are a small FI, and in some ways, that makes it easier to learn the whole system - I handle debit disputes because someone had to learn Reg E and Visa rules.