I have just installed the supercollider and supercollider-sc3-plugins packages.
 I installed SuperDirt, a quark that depends on supercollider-sc3-plugins,
which could not find parts of this extension.

I ran rpm -ql supercollider-sc3-plugins and found they were installed to
/usr/share/SuperCollider/SC3plugins/

They should have been installed to
/usr/share/SuperCollider/Extensions/SC3Plugins to be found by SuperCollider.

I resolved this with a symlink after checking SuperCollider's documentation:
sudo ln -s /usr/share/SuperCollider/SC3plugins
/usr/share/SuperCollider/Extensions/SC3Plugins

This allowed SuperCollider to see the plugins, everything worked as expected
afterward.

Here's the relevant part of the documentation that mentions where the plugins
should be installed: https://doc.sccode.org/Guides/UsingExtensions.html