We can find the class here: https://docs.microsoft.com/en-us/dotnet/api/microsoft.management.infrastructure.cimsystemproperties?view=powershellsdk-7.0.0
It is also located in windows binaries in the following path: C:\Program Files (x86)\Reference Assemblies\Microsoft\WMI\v1.0
It is also located in windows binaries in the following path: C:\Program Files (x86)\Reference Assemblies\Microsoft\WMI\v1.0