nowadays, the latest pipelines can be found on the azure dev page: in particular, the xamarin.android oss package. from there, go to linux artifacts (example). here are enough binaries to set up an install without needing to build from scratch.

the bundle we are interested in is the one labeled Linux Packages. extract that archive, then unpack the data.tar.xz of the .deb file, then copy the data to your mono installation:

rsync -v -a usr/lib/xamarin.android /usr/lib/xamarin.android 
rsync -v -a usr/lib/mono /usr/lib/mono

this should make the Xamarin.Android SDK visible to MSBuild. ensure that the variables for the SDK path are configured (this can be done in Rider settings) for example, the command line options /p:AndroidSdkDirectory=/opt/android-sdk /p:AndroidNdkDirectory=/opt/android-ndk.

this should be able to build a sample project and even deploy APKs and debug.

to test it out, try the HelloWorld project by simply using MSBuild:

msbuild HelloWorld.csproj

since we copied the frameworks to our Mono install earlier, MSBuild should be able to pick up the project SDK automatically. if it complains about not finding SDKs, pass the command line options specifying android SDK path as described earlier.