Rider can build .NET Core projects, MSBuild based projects and Xamarin Android projects.
For most project types, Rider will detect required SDKs automatically. But you can also configure SDK locations on Rider settings pages (Ctrl+Alt+S) under Build, Execution, Deployment.
For .NET Framework projects, Rider will use MSBuild or Mono’s XBuild to build your solution and projects. This means that Rider can build any project that is MSBuild based (e.g. .csproj). Again, Rider will automatically detect the location of Mono, which can also be set manually. Rider will of course automatically detect the location of MSBuild and the .NET Framework on Windows.
Rider displays a toolbar at the top right of the UI that allows selecting the current solution build configuration (e.g. Debug and Release) as well as the build target (e.g. Any CPU).