To be safe, you should assume anything that you did not create is copyrighted. The minute someone creates something (visual, auditory or written) in a fixed medium (such as a video or song) it is copyrighted by the creator.
Fair-use outlines that you may reuse certain copyright-protected materials under certain circumstances without getting permission. For more information check out YouTube's help center.
You can visit this US Copyright website to find out if a specific work is copyrighted.
Learn more about how to create original content with these YouTube creator tools!