Do you know of any resources where I can read about how banks store digital currency? Would be interesting to see how international transactions are handled, if they chunk data into months/periods, etc.

I can't say this is exactly what you're after, but this article is really interesting https://calpaterson.com/bank-python.html

Similar to what the author describes, I wouldn't be surprised if a lot of this information is generally not public.

I'm a banker. What you're looking for here is called "interbank clearing". In europe that would be SEPA[1]

But yes, most clearing is done daily. Each bank basically submits their daily flow of money to each other participating bank, and the central ACH (Automated Clearing House) keeps track of the balances. There's some processes in there by which banks can dispute charges, which is super interesting, but also way to complicated for me to detail here.

[1]: https://www.europeanpaymentscouncil.eu/what-we-do/epc-paymen...