Cygwin at its core is mainly the UNIX API implemented on top of Win32. WSL started in a very similar way, implementing Linux APIs as a new Windows subsystem, but then, for WSL2 they switched to a virtual machine approach. So, now, what you have there is a real Linux running on a virtual machine (you can even choose what distribution you want), with an extra layer for interoperability and integration with the host Windows OS. Because of this, current versions of WSL in general, work much better than Cygwin.

To be honest, that makes me a bit sad, because the Cygwin endeavor was titanic and now it is loosing all its relevance. It involved not just fully re-implementing the UNIX API from scratch but also doing it on top of an OS providing, in several cases, incompatible abstractions (for instance, fork)... if the developers only had had access to the new functionality MS added for WSL1!