A pair of database servers (leader and hot spare) behind it, although some data was in-app and shared between the two app instances (or more; two was the minimum).
I don’t think of this as particularly fancy or expensive, considering how much you have to pay for engineers and technicians. It was critical enough for >3 nines, and that means having a technician on-call.
I saw this setup at two different companies I worked at, even though the tech stacks were completely different. Whenever I price out setups for similar requirements in cloud I almost always end up with basically the same setup, just on cloud VMs instead of dedicated servers.
