Developer Guides

[scottilee]

Are there any docs that guide developers through the code?

For instance, why is the code structured under the _pytest directory?

[The-Compiler]

There are the contribution docs and the API reference (many things in there, especially plugin hooks, are relevant for the core as well).

The _pytest directory is a Python package with the leading underscore marking it as "internal", while anything that's public gets exposed in the pytest package. That's not something specific to pytest though, see e.g. https://stackoverflow.com/a/55178477/2085149 as well.