How do you purge unused materials from a blend file but not delete them?

Purged materials have to be saved in a library. Thanks

With the “Purge” button

What happens to the purged materials? Are they saved in a library Anubisbis? I would like them to be saved in a library. Thanks

They are just removed from blend file. The blender file will “forgot” this material. No save is made by Blender.
If you want to create a library, create a blender file only for it and append your material in this file and create a fake user for it to be sure the material will stay in the “library” blender file.