Not sure if this is what a "backdrop" is, but can I create an effect where a person in the video is normal but the background is replaced with a moving pattern or color or whatever else?
That's what the greenscreen technique for.
https://en.wikipedia.org/wiki/Chroma_key
In short, you need a constant color behind your person where that color will be replaced by other content, or 'backdrop' during editing (or compositing) session.
So, I take it, obviously, this cannot be applied to a video that was taken without a greenscreen initially. Thanks!
Unless you do manual rotoscoping, but that is extremely different subject and requires other compositing tools:
https://en.wikipedia.org/wiki/Rotoscoping
Too far!