Yeah, I work on systems with reliability requirements like this at a large bank.

There are multiple layers of controls and manual interventions and things, which while absolutely painful, slow, expensive and shitstorm-conjuring -- are ultimately the final authority on some failures.

For e.g, in payments -- every single settlement or clearing anomaly is looked at by a real human, and rectified/rebooked manually.

So, yeah, the stakes can be really high when you have a couple billion in memory on your server, but -- it's just a system.

And it will fail, and we plan for it to do so.