Ask HN: How do you manage documentation for large projects?

I often see projects which are massive in scale but contain little to no developer-facing documentation. Some open source projects have a contributors guide but these mainly seem to be focused on how you should style your code and format PRs rather than explaining the inner workings of the code.

I can't seem to find projects with good examples of developer-facing documentation that both explains at a high level how the code is architected, and provide specific examples of how the code does X. Take Gitea (https://github.com/go-gitea/gitea) for example, for no reason other than I'm trying to figure out the best way of storing a Git repo in a database. I can't for the life of me see how or where the contents of individual files are being stored.

I'm always amazed at how many quality contributions the community can make to open source projects of this scale. I also wonder how much time is being wasted on learning the codebase by reading the raw code vs. a distilled representation that well-written docs would give you.

So I'm here to ask - how do you do documentation so newcomers can get a foothold on the codebase?