Domain aliases allow you to add additional domains that share the same mail server and email accounts as the primary domain. You will not have the ability to change any settings for the aliased domains because all settings (including email recipients) will mirror the primary domain.
Domain aliases are not just limited to domain extensions, but can be used in any case where secondary domains have email configurations that are identical to the primary domain. So, if example.com was the primary domain, but another-example.com also shared the same email server and email users, it could be set up as a free domain alias in SpamHero.
To add a domain alias, login to your SpamHero account and go to the Settings > Domain Aliases page. After you have added the domain alias there, wait 5 minutes and then update your domain's MX records to use the ones displayed at the bottom of the Domain Aliases page.