Depends on what you want to do with it.

Most archives I’ve worked in have photo and video reserves which are freely available to researchers who visit the archive— some post material online, but that’s pretty limited and usually low quality.

However, every archive I’ve dealt with, so primary French state with a few private American ones, have commercial exclusions. You cannot use the material for anything commercial— in the case of photos in a published book, there is a fee that often times needs to be paid and print restrictions on how many time the image can be reprinted— I’ve seen a couple academic books that had runs of only 200 copies because the author/publishers really wanted a particular image or series and that’s the only way they could get approval. Things can get a bit more complicated if say the material is held by the archive as a private collection and you need not just the approval of the archive but the estate/family whose material it is.

Really though, the best way to find out about the availability of archival footage is to contact an archive whose holdings you might be interested in checking or who holds material you already know you might want to see.