How to replace a character in a video?

Question

Video processing

Let's say I want to have these characters instead of these characters in some clip about Rapunzel and Eugene: and so that these inserted characters repeat the action after Eugene and Rapunzel.

Answer 1

You contact the author of the clip, get the source code for the render from him, change the models in the source code, start the render of the clip from scratch, and you get it.
Just replace it in an already finished clip - well, you can paint on each frame pixel by pixel, it will probably take a couple of years. For which you can try to study programming, mathematical analysis, study how neural networks work and try to partially automate, but the result will not be very good.