For anyone curious (as I was) the basic difference is that WSL1 implemented the Linux syscall table directly whereas WSL2 actually runs Linux on top of some virtual drivers (hypervisor).
WSL 2 runs a full Linux kernel under Hyper-V. There are some out-of-tree or staging drivers included in Microsoft's Linux kernel derivative and they publish their kernel sources at https://github.com/microsoft/WSL2-Linux-Kernel.
i routinely upgrade my WSL2 kernel. Now on 6.6.87.1. Personally, I love WSL2.