New to mxroute so setting up for the first time 
From https://mxroutehelp.com/index.php/2019/11/03/how-to-add-domain-alias/ I gather domain alias allows me to send/receive email from user @ aliasdomain (source) and see the emails in maindomain (target). I am assuming aliasdomain would need the DNS for MX, SPF/DKIM records setup too just like maindomain? It isn’t specifically mentioned in the article but I assume that is the case?
You’ve got it right. On the back-end a domain alias is literally a symlink. So like /etc/virtual/domain.com has in it your alias file, password file defining the email accounts, exim filters (“spam filters” in DA), autoreplies, DKIM public/private keys, limits, etc. And then you add domain.net as an alias and it just symlinks it to domain.com.
Cheers thanks for the confirmation. Might want to add to that article the same DNS record treatment is needed for the alias domain too. It might be obvious to experienced folks but might not be for noobs