How to extract multiple camera's from one source file containing multiple angles side by side?

I'm new and might not use the correct terminology, which also makes searching for this difficult in the community; apologies.

I have one source file (mp4) that has a huge resolution (5760x4040 I think). Within that file are multiple camera angles (5) and other screen captures (3) side by side. See my sketch attached to this post/ below. Important to realize is that the one file has thus all camera images in “one track”.

(Scetch - all this is one frame in one file having the camera's side by side.)

So my question, what is the best way to mark the “camera’s” in that file so I can use them with multicamera editing? So how to I mark “area’s” in the one file as separate camera’s? How'd you handle this?

Additional question: if I do end up needing to extract them to separate sequences (and later add them together to do a multicamera edit); is there a way to save the “stencil”? So that I can easily extract the sequences without memorizing the x,y-coordinates to crop/ pan each camera on each new file.

Thanks for any tips and pointers to the correct solution and or terminology :).