When I worked in a different career I worked with big banks and did disaster recovery tests (we didn't run the tests we just had equipment that the tests sort of centered around). We'd basically cut of a data center for a weekend and they would run from another data center that was supposed to have all the data there too. We'd even move their check processing to a backup site and they'd truck full truckloads of paper checks to a backup site.

They were legit tests at the offline site too too, they'd power down equipment, and power it up and we'd fix what didn't come back up. Even the data centers would be fully powered off to test.

At least at that time those banks did not skimp on those big tests and it was a big effort and pretty dang well run / complete.