Creative Commons is a copyright license that is a simple, standardized way for creators to give users permission to share and use their creative work. So if a creative work states that it is licensed under the Creative Commons, it is generally free to use. You should always double-check policies before using though, as there are different types of CC licenses.
Learn more: https://creativecommons.org/licenses/by-nc/4.0/
Copyrighted works end up available through the Public Domain once a certain amount of time elapses, or sometimes it is because of other legal reasons. Sometimes it is difficult to tell what is considered in the Public Domain because since it is not owned, there is no one to ask, so some research may need to be done.
Learn more: https://creativecommons.org/public-domain/
