Is there a way to replace black or white areas of footage with other footage?

I have some footage (footage X) that is abstract shapes moving around, all of which are either pure white or pure black. I hope to be able to replace the white with footage A and the black with footage B. I haven't been able to figure out how to do it though.

Is there a way to turn footage into a mask? If there is, then i could just use footage X as a mask for footage A, then use inverted footage X as a mask for footage B.

I am still pretty new to this so I am sorry if this is turns out to be a basic question. Any help at all, even if its just giving me some keywords to search for on youtube, would be greatly appreciated.