How do I create a template email in Mail?

Mac’s Mail app doesn’t really have this capability. Consider using Pages to create a document with such a template and then emailing copies of the document.

Mail does allow you to create a signature.

More details are here.

Mail User Guide for Mac - Apple Support

thomcorr wrote:

How do I create a template email in Mail? I need to send out emails with a specific graphic at the top each time.

https://discussions.apple.com/thread/250090975

Sounds more like a signature—here is the best tutorial on HTML sig:

How to Make an HTML Signature in Apple Mail for macOS ...