Understand site/domains

I can’t seem to wrap my head around using and setting up multiple sites. I have two separate use cases and would like opinions on what the best action would be. I did use the docker install and running 13.9

1.) I would like to use the default shopping experience with erp next. When setting up the instance I used erp.domain.com. But for the “home” and shop pages I would like the domain to only be root (Or possibly differentdomain.com) . Would this be setting up another site on the frappe side, or just nginx configuring?

2.) I have a company the owns and manages other companies. Would this be more setting up sites within frappe, or have separate instances?