In WSL1 Microsoft only ships an implementation of the Linux syscall interface, the actual userland comes in the form of a tarball inside an appx package from Canonical, through the MS Store.

I don't exactly know how WSL2 works, but the main difference is that they now use an actual Linux kernel inside a lightweight VM, and that kernel's source is on GitHub. Userland still comes from appx-wrapped tarballs from Canonical.