Help me understand Remote Extension Host builds and how to use them, please

[HoboDev]

I'm trying to understand what I can do with the reh builds. (and how)
In some comments I've seen they are for servers. Also this repo needs them installed manual for some BSDs: https://github.com/jeanp413/open-remote-ssh
And specific builds like the alpine-reh is mentioned in the issues.

I've looked through the documentation, the website and have been searching for about 3 hours...
So, what exactly does this do? How does this work with open-remote-ssh?
Is this some kind of devcontainer-esque solution?
Where can I find the documentation for that?
How do I use it?

Any help is very much appreciated.