I've worked at many big banks and corporations. They are all held together with the proverbial sticky tape, bubblegum, and hope.

They do have multiple layers of redundancies, and thus have the big budgets, but they won't be kept hot, or there will be some critical flaws that all of the engineers know about but they haven't been given permission/funding to fix, and are so badly managed by the firm, they dgaf either and secretly want the thing to burn.

There will be sustained periods of downtime if their primary system blips.

They will all still be dependent on some hyper-critical system that nobody really knows how it works, the last change was introduced in 1988 and it (probably) requires a terminal emulator to operate.