Copyright protections are applied to creative work that is set in a fixed medium. You will need to be granted permission before using the material in a way reserved for the copyright owner, although many exceptions exist, and you do not always need to seek permission. Copyright is automatic, and begins the moment an author captures their work in a fixed medium. Copyright does not apply to facts or ideas.
In general, works created on or after January 1st, 1978 have copyright protections for the life of the author, plus seventy years after the author's death. If there are multiple authors, the term lasts for seventy years after the last surviving author's death.
Works for hire and anonymous or pseudonymous works: the duration of copyright is 95 years from the publication or 120 years from creation, whichever is shorter.
Works created before 1978 have confusing and differing copyright lengths depending on a variety of factors. For help understanding the status of a pre-1978 work, see the Copyright Term and Public Domain in the United States link below.
Once copyright protections expire, the work passes into the public domain.
Online Content is not automatically copyright free.
A common misconception is the material found online (images, videos, online documentaries, etc) are free to use. These items are still copyright protected and require you may need to seek permission before using them. Even if the item lacks a copyright notice, it does not make it automatically in the public domain. There are many online sources of free-to-use material. Be sure to use these websites if looking for things like images to use in presentations.
While the YouTube platform is free to access and use, this does not apply to the content. You should also seek permission to reuse material in presentations. Copyright permissions may be listed in the description or require you to contact the channel owner. Linking to a YouTube video for individual viewing is often the best way to share YouTube content.