Copyright of materials used in a video
Very broadly speaking, you have a much easier experience obtaining permission if you are not making money from someone else's content.

Most copyright laws recognise 'fair use' -- a small quote or a clip that is proof of someone else having said something.  It is customary and advisable to put such clip in a box or window to show that the content is being referenced.  Fair use for a small portion of text or video is easily established than a picture, for a picture is the entire content.


For images, search engines usually have a license filter in the search. Usually, it errs on the side of caution. Unfortunately, you have to contact the original agency or person and obtain permission to use.
Youtube now has content signature comparison and will warn you that you are using someone else's content but will leave it up to the copyright owner to enforce it.


So, if it is does not fall under 'fair use' and is clearly someone else's copyright, you will have to track down the owner and obtain permission.