They have a stand-in system that uses a separate codebase and separate cloud provider. They have a blog post about it here: https://monzo.com/blog/tolerating-full-cloud-outages-with-mo...
The stand-in system has a limited subset of features, i.e. no bells and whistles, but enough that you can get any actual banking done.
(Not an ad / I have no connection to Monzo, I just had some appreciation for a bank doing this properly as normally banking IT is a disaster. Plus the idea of keeping a separate codebase for almost an entire bank's functionality seems very invested.)
I have always got bad vibes from Revolut...