Assume that work you find on the Internet is under copyright.
Copyright exists from the moment a work is fixed in a tangible medium. Some content may be in the public domain or be licensed for open use, but don't assume that is the case.
If you are doing a presentation for a course assignment, linking is ok! If you are doing a presentation for a course inside Canvas so that your work is available only to the students in your course, it should be ok! Make sure the work is password-protected and only available for the time necessary.
If you want to ask for permission to use a work, contact the copyright holder. (One question to ask, "Are you the copyright holder who can grant permission to use this work?") Save any email or correspondence in case of a dispute.
Respect your instructor's work! Follow your instructor's wishes about sharing course content.
Remember that when you write papers for your courses, projects, etc., you are the copyright holder for the work you created.
Whether the work is under copyright or in the public domain, always cite your sources!