As long as you use the "authenticated domain" after the @ you can pretty much do what you want with the so called localpart (everything before the @). The same is true for "registered domains" but those lack important deliverability features.
You, of course, have to stick to standards for valid email address formats but otherwise this is pretty flexible. Read this documentation to understand the differences and implications.
The address you create in from address management or in sender profiles can be understood as a virtual address that only exists for outbound branding purposes .
You cannot send emails to this address, however. Inbound email is only processed by marketing cloud when the reply function of an email client is used. Replies do not rely on the address itself but on specific reply-to path (tied to the send job and subscriber) that is sent in the header of an email you send with marketing cloud. At this point it gets very technical but this website has some easy to grasp explanation of this.
Reply Mail Management is used to configure this behaviour in marketing cloud, e.g. to define real addresses where those replies should be forwarded to. This can be account wide or specific to a sender profile.
As there is quite a bit that you can do with Reply Mail Management you should read the documentation to get familiar with the concepts and options.
