Thanks JessieZhang-2116, but you were just regurgitating sites that I had seen and not been able to comprehend.
Us newbies need a little more hand-holding.
1) Set the build target to AAB in the project file.
2) Uncheck signing from the APK signing option in the project options.
3) Archive the build using VS normally.
4) Perform an Ad-Hoc Distribution signing the AAB that way.
5) Upload in the Google portal normally.
With a little help from a bunch of searches, but predominantly, https://learn.microsoft.com/en-au/xamarin/android/deploy-test/signing/?tabs=windows, these are my instructions, which seem to work:
1) Right click on Android project and select Properties > Android Options > Android Package Format > Bundle
2) Right click on Android project and select Properties > Android Package Signing > uncheck Sign the .APK file…
3) Right click on Android project and select Archive
4) After Archive has completed, click on Distribute and select Ad Hoc. In the App Details Signing Identity Window, click on the plus (+) button to create a new Java Keystore or select an existing one. Resave the new .AAB. In my case, to
C:\Users\grime\AppData\Local\Xamarin\Mono for Android\Archives\2020-12-11\Fartz.Android 12-11-20 8.38 PM.aabarchive\net.grime.apps.fartz.aab
5) Go to the Play Store Console site. Add or select your app. Drop your bundle (aab) in the designated space. If there are no errors, click on Save at the bottom, then Review Your Release.