I have not played with Blender in a while, and if I did know how to do this, I forgot.
I got a video that has some "black screen" behind a person, and I am using that as a "green screen" and place some stock video as the background instead of just seeing black as a background.
I've almost figured out the particulars in taking the videos and exporting them as frames then using the frames to do this. But I have seen in youtube on this that they say one can use movies strips, but they never show how that is done, and just show the use of frames instead. this method takes a lot of time having to frame out every sequence I want to use and if it does not look right go back and get some more frames if I have not framed out the entire movie.
using two movies and placement of the background movie where I want it to become the background for the duration of the blackscreen then go into compositing and use the nodes to show the movies. this seems sound like an easier, and less time consuming.
the two movies I got sowing, but the place for the background is not in the correct place in the node to tell it what frame to start, and end, as well as the foreground movie using the movie node. so when I render the background does not move along with the foreground movie.
when I switched it to the image node, setup some arbitrary amount of frames , start frame 1 , and offset where it is at on the film strip, then match that up with the foreground movie using the same image node setup, the backdrop shows the two images like it should and in the render results it does not. It just renders the background movie in sequence, and moves it alone fine, without overlaying the foreground image on top of it. they are both selected to cycle and auto refresh.