Put a video in the background of another video

I would put a video in the background of another video.
I saw an explanation but I do not understand.
Could you explain that more clearly? I have difficulties with English because I do not master it, that’s why I have the translation translated by google translation.
Thank you for your help.

Thanks for the video that is super well explained.
On the other hand, what I wanted to do is a little different.
It is to put two videos at the same time but that one is a bit transparent to let see the other one passes.
For example, I am in a wood, there is a squirrel and on another video I have a butterfly that flies.
I want to overlay the two videos to see one over the other.
Is it possible ?
Thank you in any case for your very valuable help.

Provided the ‘butterfly’ video has been shot against a green screen or has a solid background colour that you can set as a chroma key, then it’s possible using methods described in the video.

Also see