MacOS has been supported already for years, for plug-in development via RhinoCommon. Or did you mean the Rhino C++ SDK.
I integrate Cycles into Rhino to power Raytraced (since Rhino 6) and Rhino Render (since Rhino 7). Cycles itself is C++, the integration plug-in RhinoCycles is C#.
You can use GitHub actions to build for all supported platforms. For instance our rhino3dm, which is our wrapper around OpenNURBS for Python (rhino3dm · PyPI), Javascript (rhino3dm - npm) and .NET (NuGet Gallery | Rhino3dm 8.17.0) is fully built using GitHub actions for all the platforms you can see, and for multiple versions - the version/language/OS matrix is quite large.
You should have a look at GitHub - mcneel/Rhino.Testing: NUnit dotnet unit testing for Rhino3D to set up testing to run within Rhino. You can refer to Automated testing with Rhino ✅ to find out how to do that with GitHub actions as well, including using Rhino in a GitHub action.
As mentioned before, RhinoCommon requires Rhino to run properly. See the previous paragraph with link to post to set up unit testing in GitHub actions so that you can use and test RhinoCommon.
See the earlier paragraph on setting up testing in GitHub actions.