Setting the Default Workflow User
The Default workflow user's email address is set, by default, as the Sender of all email alerts send through workflows.
It can be defined to be any licensed salesforce user in your organization - and typically is the System Administrator.
Default workflow User (Classic)
The majority of the emails in the system are sent from whoever you designate as "Default Workflow User" (Or you can set an organization-wide email address as the 'from' for your emails.
We pre-set this as your system administrator. But you can make any licensed Salesforce User in your system the default workflow administrator.
Default Workflow User (Lightning)
Go to Setup / Process Automation Settings