[alex-nt]

I might have a bit of insight here. I work in a bank that has a lot of history. In practice, the IT infra is never truly merged (it takes 10+ years to migrate some components), not the functional components at least. There are bridges to connect some things so that reporting is deemed acceptable for the central bank/regulators.

If you come to one of the big banks and just look at the technical components, you can delineate the history of each merger/acquisition from that, going back 30-50 years.

A thing that shocked me: a lot of systems have a weird nationalistic twist to them (at least the old ones), so if you work in non-US, you will see systems that do not speak english in their API, but french, dutch, german, etc. all in the same bank. This is...problematic, especially since they usually use weird text encoding and not something like UTF-8.