I'm not completely sure what it is you want to know, but basically there are two pieces to linux container tech (Docker and others). The first is a set of Linux kernel features that lets us isolate various aspects of processes in separate namespaces. The second is layered images, using filesystem features like overlayfs and copy-on-write to avoid having to duplicated everything. These two features of the Linux kernel are the real "container technology", docker and others are basically just user interfaces for these.

A link about namespaces:

http://ifeanyi.co/posts/linux-namespaces-part-1/

A nice little introduction to overlays/etc:

https://jvns.ca/blog/2019/11/18/how-containers-work--overlay...

And if you really want to learn how it all works, write your own "rubber-docker" in Python:

https://github.com/Fewbytes/rubber-docker