Ask HN: What's the best way to learn how Claude Code/codex works?

I've been using claude for a while, but I'm interested in learning how it all works.

* How does the terminal work as an app?

* How does claude code know to spawn different agents?

* How does claude code know which tools to use for each prompt?

I'm just curious about how it all works. The best idea I have at the moment is to read codex's repo and learn it that way. But I want to check if someone went through this path or as other ideas?