When previewing a template you will see your logo image and phone number by default.
That Header and footer cannot be edited as it pulls from your Company Phone Number and Company Logo found under Company > General > Logos, Marketing, and UI.
If you want more customization you can turn off the default Header and Footer, but PLEASE NOTE: You will have to edit each template to have a header/footer if you want there to be one.
To do this you can change Default Email Wrapper Template from Original to Plain. Which will make it look like below. This allows for full customization but leads to more work to change each template.