Sounds like your friend is just blindly following other people without any better understanding of mapping than you have.
The way I like to explain it. "Mapping" just means a correspondence. It is often used in this abstract way in other areas such as mathematics.
Just as a location on a road "map" corresponds to a location in the world.
In docker, a container path is mapped to a host path. That is just establishing a correspondence between a path on the host (Unraid) and a path inside the container.
How you choose to map those host paths to container paths is up to you, but you need to understand and pay attention to it, since many docker applications will need you to specify paths within the application. Downloading applications like nzbget, for example. And there are times when you might need certain docker applications to communicate paths to other docker applications, radarr and nzbget, for example.
Port mappings are similar, establishing a correspondence between ports on the host and ports within the container.
If you understand mappings, you are most of the way to understanding how to setup any docker.