How are the user variables mapped in the engagement suite messages?

When you create an engagement message, you can add user variables such as firstname, lastname, email and phone number. 

These fields will pull in the corresponding data from the user profile when sent.

But how do we determine which user profile to use?

This is decided by the option you've selected under 'from':

So this will either be the advert owner, vacancy owner, the user who actions (triggers the message), or a specific user chosen from the list.