System design in practice: how to actually understand and reason about design decisions in practice?

So we all know at this point that if somone wants to crack MAANG or any top tier product companies then learning system design is inevitable. And there are infinitely many resources to read and learn about it from an interview standpoint. Usually this would be like how would you design a product X and why specific tech stacks are chosen. What I would like to know is how people actually make such design decisions in these organizations. It would be great if it can be elaborated on the following points: - The processes involved from idea to implementation. - Who all are involved and the role or contributions made by each person from junior to senior engineers, PMs and researchers and any other people relevant. - What are the aspects considered when implementing and what all metrics taken into consideration especially if it is major thing that can effect their current operations. - Incase the implementation fails, then what happens (fallbacks, rectifications etc)? - How a specific technology is decided upon including cases where creating a custom solution is considered? Any incidents or real life situations that can be shared would be great but I can understand if specifics need to be left out due to NDA/ Anonymity.