That sounds even more insane to me, but I guess most of that code does not really touch financial transactions, otherwise it would be a nightmare being responsible to verify that.

Ruby code touches financial transactions. Card payments were migrated to Java when I left in 2022. Non-card payments (e.g., ACH, checks, various wallets) were still processed by Ruby.

PCI-related/vaulting code lived in its own locked-down repo. I think that was a mix of Go and Ruby.

Once you have the foundations in place for account balances and the ledger, processing a payment isn’t that daunting. Those foundations, however, took a lot to build and evolve.