Thanks to @obj with whose help & guidance I could finally succeed in installing dump1090-fa and piaware on Pi 4B running Ubuntu Server 20.04 arm64.
Thanks also to @postl and @mikkyo for their contribution.
I have created a separate thread for the finished procedure. This will enable users to get the complete and final howto without need to brows through lot of posts.
Howto Install Piaware 3.8.1 on Ubuntu Server 20.04 arm64 / Pi4