Hello and welcome to Horos!
Unfortunately the Plugins->Plugins Manager is not listing some of the commonly available plugins available for Horos. But they seem to be found here :
To install, download the plugin zip, unzip, and then open the .horosplugin file. Horos should then open to install it where it needs to go.
And web searches may turn up a lot of links to outdated or stale plugins that no longer work in the latest releases of macOS and Horos. You can also search for OsiriX plugins as they usually also work with Horos.
Some I the best I know of are from Imaging Biometrics (
https://www.imagingbiometrics.com) handle perfusion and diffusion processing. Full disclosure: I am a developer working on many of these.
Todd Jensen, PhD
Jensen Informatics LLC