I’m sure you can appreciate that we can’t talk openly about the details of our financial crime systems however I can say that our financial crime systems stop more than 50,000 transactions a month.
Here is some “under the hood” stuff:
https://webviews.monzo.com/compromised-card/suspicious-transaction/
https://webviews.monzo.com/compromised-card/magstripe-technical-fallback/