Now I’m looking for the Harrison XT plugins that I like to use in previous projects, most of which I have paid licenses for.
Apparently these were not installed.
Could this be because Ardor was included by the distribution (ZorinOS)? Should I reinstall Ardor from the installer?
Those plugins are proprietary (closed source). Harrison has granted ardour.org to redistribute their binaries with Ardour.
If you want to keep using your distro’s version of Ardour, the easiest way to get the XT plugins is to install the official Ardour version (demo is fine), then copy the plugin bundle /opt/Ardour-7.5.0/lib/LV2/Harrison.lv2/ to ~/.lv2/Harrison.lv2/ (and then uninstall the official binary /opt/Ardour-8.0.0.uninstall.sh)