Gnome Tweak Tool is no longer allowing to move the Gnome Application Menu out of the Top Bar on Ubuntu 20.04
You can remove the application menu using the Remove App Menu extension. You will not miss any Application Menu items (e.g. "Preferences, ...): since a few versions of Gnome Shell, these moved back to where they belong: the application window itself.
Next to removing the application menu, much more about the topbar can be configured with the Activities Configurator extension, which is around for a long time and very well maintained.