Short answer
HubSpot's KB: use personalization tokens to set the From name, address and signature to the contact owner, for example choosing owner.email in the From address dropdown. Test emails always come from noreply@hubspot.com, and the signature appears in a test only if you send it as a specific contact. Each owner edits their own signature.
1. From line
The email header in my demo portal.
2. Contacts with no owner
Set a default From for contacts without an owner.