Ask HN: How do you find the "why" behind old code decisions?

Curious how teams actually handle this in practice.

When you’re modifying existing code and need to understand why a decision was made:

Where do you look first?

How often do you fail to find a clear answer?

What do you do when the original author is gone?

I’ve been running into this more often lately --digging through PRs, Slack threads, and half-written docs just to piece together context, and I’m wondering how common this really is across teams.