Screensavers are employed within System Preferences.
Apple's own Screen Saver protocol works by means of placing one's protocol formatted composition within the Composition Repository Library/Screen Savers, either one's Home folder ~/Library/Screen Savers or /Library/Screen Savers.
As detailed earlier, one can combine protocols within the same composition, so it can be used as both a ScreenSaver and a Visualizer. So long as a copy of the composition is saved within each appropriate Composition Repository, then such a composition will be available both within iTunes to visualize audio and as a screensaver too. One cannot combine Image Filter protocol with any other type of protocol based composition although Screen Saver based compositions can combine any other protocol than Image Filter.
There is a another location that acts as part of the Composition repository, which is /System/Library/Compositions - reserved for core system compositions, given that these are exploited by Finder and other default applications, it is advisable to leave these alone, although there is nothing to stop one examining these compositions and exploiting them either for all users , or just for oneself.
You just have to remember to save them outside of the System/Library/Compositions folder.