If you're at the repo level, then read the commit messages, release notes, and issues and comments linked to a pull-requests. They will contain the rationale and discussions on technical decisions and the tradeoffs behind them.

This sounds right up your alley: http://www.aosabook.org/en/index.html

Shameless plug, while this does not have full articles explaining decision process, I started tracking design decision with a short summary and the date those choices were introduced: https://github.com/wiringbits/scala-webapp-template/blob/mas...

This has been helpful to my team because there are times where something does not seem right, or, it just seems a weird choice, documenting the reasoning can help new team members to understand this.

In retrospective, I should have started doing this many years ago.