The glue that holds it all together? The Database. Banks are just databases with some retail branch offices. :-)
I used to work in a (US) FDIC (our bank regulator) regulated environment - we were technically a bank, and when I left we were adding some obviously "retail banking" kinds of things - checking accounts and mortgages, for instance. I got out ahead of the mortgage-backed debacle, but it was pure luck.
We were moving to a sort-of microservices model, but mostly using MQseries Transaction Integrator (formerly NEON, now Who Knows What). Systems were 90+% MVS (IMS and CICS+DB/2)
Also, am I the only one who saw "Cassandra" and immediately thought "Free money for everyone!"?
A quick look at their website and wikipedia suggests they're basically a prepaid debit card business, maybe even just a reseller, who recently added checking accounts. In the US you could outsource all of this to First Data Corp. Many, many small banks do just this - "The Bank of Southeastern Crud County" (over 500 customers!) provides a building, tellers, and a few million in startup capital. First Data does the whole back office, even prints junk mail if you want them to. It's conceivable that Monzo's entire system is just/mostly interfaces to vendors.
So, yeah, if they're 95+% virtual, this would work. FDC (just as an example - I don't endorse them) would provide the database of record and even Cassandra would then be survivable. They already mentioned the big card providers, plus Apple Pay and a few others, so perhaps many of their 1600 systems are marketing and other ancillary activities. Evidently it works, but given how much money they've had to raise I doubt it works cheaply.