What is an email alias and how can I log into my eXp email
An email alias is simply an email forwarding address. The mail server simply forwards email messages addressed to an email alias to another specified email address.
In your case, when an email is sent to firstname.lastname@exprealty.com it is automatically forwarded to your personal email address. Since this is an alias, no login information is needed as there is nothing to log in to except your personal email address.
To send email from your @exprealty.com address (rather than from your personal email), you'll need to configure Send As on your personal mail client. Setup varies by mail provider and agent type — use the walkthroughs here: