If you'd like to use the same template in multiple ESP email campaigns, you can download the email template from the ESP dashboard and reuse it easily.
For this, all you need to do is navigate to Email → Campaigns → choose the relevant campaign → navigate to the tab Template & Style → and select the icon for Download as a template on the right hand side.
This will start a download of a JSON file of the template to your computer.
Afterward, during the process of creating a new email campaign under Email → Campaigns → Create campaign, you can import this file under the tab Template & Style: