Just a small guide kinda thing on how to develop for Unity on NixOS, at least my journey in this space. If you find anything else to add or any steps I missed here, please mention in the comments!
Rider needs access to MSBuild, .NET, and Mono in order to do code analysis. I found that manually setting the paths inside of Rider never worked correctly, so I instead wrapped Rider in a FHSUserEnv.
I have a workaround to allow rider to be detected by the Unity Rider plugin, which allows you to set Rider as the external editor in Unity, and now works with Rider's Advanced Unity Integration!
Unity Hub downloads and manages Unity verisons and projects.