

For free stock images and videos https://search.creativecommons.org/ is a metasearch of creative common contents, https://www.freepik.com/ has free stock images but it has also introduced ai, https://pixabay.com/ also has many royalty free images. Check their license terms before using.
Using clips from other youtubers, historical footage etc would fall under fair use law (at least in usa, check your local laws) if you add your own content to it. There are detailed videos on fair use from youtube creator academy you can watch them.
Sovereignty is British occupation