When reading the reference to "internally maintained technology" as opposed to open source, at an investment bank, I was reminded of this eye-opening HN post, "An Oral History of Bank Python", https://calpaterson.com/bank-python.html / https://news.ycombinator.com/item?id=29104047 . Which goes into quite a bit more detail (perhaps on similar/same things?) than an intern with an NDA would be comfortable sharing.

Pretty wild stuff.