I wish that were true.
In my experience, most of the time they're not doing the things you talk about -- major architectural decisions don't get documented anywhere, commit messages give no "why", and the people who the knowledge got socialized to in unrecorded conversations then left the company.
If anything, LLM's seem to be far more consistent in documenting the rationales for design decisions, leaving clear comments in code and commit messages, etc. if you ask them to.
Unfortunately, humans generally are not better at communicating about their process, in my experience. Most engineers I know enjoy writing code, and hate documenting what they're doing. Git and issue-tracking have helped somewhat, but it's still very often about the "what" and not the "why this way".