Adding a user just adds a guest user. Once you add the User if you register them it sends an email.
osTicket can be configured a number of ways.
For receiving email it can pick up email via POP or IMAP or direct Piping to the system.
I would say that POP or IMAP is the most used, but thats more a speculation based on the number of posts I have seen regarding them vs people using piping.
For sending email it uses SMTP, or PHP mail. I would guess that most people use SMTP these days but again that is my speculation.
You can find more information here:
https://docs.osticket.com/en/latest/Getting%20Started/Email%20Settings.html
If you have any more specific questions I'd be happy to answer them. Happy Friday!