Great reference.

To this date, one of the best well-documented code is jQuery: https://github.com/jquery/jquery/blob/main/src/core/init.js

I learned a lot from looking into the code.

Agreed although I'm somewhat confused since the three js source code is JavaScript not TypeScript.

It is auto-redirecting to the @types/three package and getting the types from DefinitelyTyped. You can see that in the breadcrumbs, but it might be something that could be better highlighted (and maybe even redirect the URL to make even more obvious to developers).