How does MultiSite work?

I was reviewing the multi-site/network documentation but I’ve got some questions for clarification.

I understand that you can use directories. I am hosting with BlueHost and the way they “addon domains” creates directories. For example, https://www.mainsite.com/addondomain where the directory represents the add on domain.

So my question is this, I would install WordPress MultiSite in the main home directory of the server. Then, would my WordPress network consist of the other domains, or only the subdirectories? Would I be able to accesss the network then on https://www.mainsite.com (netowrk home) and https://www.addondomain.com?

I guess I’m asking really if the network/multisite supports multiple domains, so long as they map to sub-directories.

Thanks,

Jon