MSBuildSdkExtras | Extra properties for MSBuild SDK projects | Form library

Visual Studio 2017 Update 6 (aka v15.6) includes support for SDK's resolved from NuGet, which is required for this to work. VS 2019 is recommended.
Create a new project .NET Core console app or .NET Standard class library. With your existing SDK-style project. With the templates in the repo's TestProjects folder.
Replace Microsoft.NET.Sdk with MSBuild.Sdk.Extras to the project's top-level Sdk attribute.
You have to tell MSBuild that the Sdk should resolve from NuGet by Adding a global.json containing the Sdk name and version. Appending a version info to the Sdk attribute value.
Then you can edit the TargetFramework to a different TFM, or you can rename TargetFramework to TargetFrameworks and specify multiple TFM's with a ; separator.
It will only work with an IDE that uses the desktop msbuild (i.e. Visual Studio) and the target Platform SDKs which are not cross platform.
When using JetBrains Rider, you will need to point to your desktop MSBuild in your settings (Settings > Build, execution, deployment > Use MSBuild Version)
When building from the CLI, you must use MSBuild.exe. dotnet build will not work for most project types.
It might work in Visual Studio Code, but you have to configure build tasks in launch.json to use desktop msbuild to build.
You must install the tools of the platforms you intend to build. For Xamarin, that means the Xamarin Workload; for UWP install those tools as well.
Make sure to use TargetFrameworks instead of TargetFramework, even if you're only building a single target framework. I am piggy-backing off of its looping capabilities.
Set the RuntimeIdentifiers property to valid RID's (full list), separated by a semi-colon (<RuntimeIdentifiers>win;unix</RuntimeIdentifiers>).
For the TFM's that you want want to build separately, set the ExtrasBuildEachRuntimeIdentifier property to true.
You must use the Sdk="MSBuild.Sdk.Extras" method for this. Using PackageReference is unsupported for this scenario.
While the Visual Studio context won't show each RID, it'll build for each.
The Extras defines a preprocessor symbol for each RID for use (win-x86 would be WIN_X86 and centos.7-x64 would be CENTOS_7_X64). Dots and dashes become underbars.
The default path for per-RID output assemblies and symbols in NuGet package is runtimes/<RuntimeIdentifier>/lib/<TargetFramework>.
RuntimeIdentifiers can be set per-TargetFramework using a condition on the property. This lets you have multiple TFM's, but only some of which have RID's.