𝘉𝘢𝘤𝘬 𝘵𝘰 𝘉𝘢𝘴𝘪𝘤𝘴: go read Cal Paterson’s “An oral history of Bank Python”. It describes a class of software systems that run inside large investment banks, and it is a instructive case study on software architecture. You will probable start reading and thinking “wow, this does not seem right”. Until you realize the constraints that produced them.

Every system runs on limits, whether you name them or not, what’s allowed to change independently, what has to stay coupled, what the system will simply refuse to do, what trade-off you’re accepting today so you don’t have to relitigate it tomorrow. Good architecture isn’t the absence of constraints. It’s choosing the right ones, early, on purpose.

https://calpaterson.com/bank-python.html