Found Footage
See Found Footage selection on Films to Watch section of films - link here
A collection of films that use existing works, or materials to make something new.
Also see Archive page on Documentary section, with links to Archives for Education website through which you have access to tons of archive footage that can be used legitimately within your projects. Also good resources on that site too.
Also check out new Adam Curtis film series that starts today, 11/2/21 on iPlayer