A email address is a completely separate email account with a unique username and login credentials. This means that you would have to manage two separate email accounts and log in to each one separately.
On the other hand, an alias is an additional email address that is associated with your existing email account. Emails sent to the alias will be delivered to your primary email inbox, and you can choose to send emails from the alias as well. This can be a useful way to manage multiple email addresses without having to log in to separate accounts. This is also a good way to hide your true email address when signing up for some online services.
A third option is the shared mailbox, which is a mailbox that multiple users can access using their own login credentials, without the need for a separate login. The shared mailbox is a self-contained mailbox that can be accessed by multiple users simultaneously, making it a useful option for teams or departments who need to collaborate on email messages and have a shared email address.