How to deal with two containers(both are servers) in docker?

Question

Docker

I'm trying to get into the docker, although it's very difficult due to the lack of basic knowledge on this topic.
I want to make a sandbox - the idea is that I will interpret the code in an isolated environment - in a container. That is, in fact, I will have a site-application, inside of which there will be a sandbox with a container. That is, in fact there will be two servers, I can’t understand how they will work together, how is this done?