The sc3-plugins are a collection of unit generators (UGen), developed by the SC community. For using the full potential of SuperCollider, it is recommended to install them alongside every SC install:
The sc3-plugins also contain the SC-HOA UGens, which are needed for working with binaural and Ambisonics in SC.
Extra classes are organized in so called Quarks:
Some addons for SuperCollider require plugins AND extensions. Quarks can be added via code. This example shows how to install the SC-HOA quarks (https://github.com/florian-grond/SC-HOA):
Quarks.install("SC-HOA") // SuperCollider will automatically install the Quarks here: Quarks.folder; // execute to see the directory
Another way is to use the Quarks GUI:
The sclang needs to be restarted after installing Quarks and extensions. When doing so, the class library gets recompiled and newly installed classes or unit generators are available.
After restarting sclang (recompiling the class library), you can check whether a Quark (SC-HOA) has been installed properly by checking for related help files, which will be installed: