If you use HTMLservice templates, you may want to centralize useful ones and use them in multiple projects, just like you can with library scripts.
All you need to make this work is a library with your template file and a one line function in your library. Here’s a snippet from Displaying analytics data on site pages, where I have my template in a library file, as opposed to the project I’m working on.

In my library, I have this function

function getInstrumentationCode () {
  return HtmlService

and an html template called ‘instrumentationTemplate’

Back in my main project I call it like this

// create the page
    return HtmlService

Job done. Now I can use the same template across multiple templates.

For more like this see Google Apps Scripts Snippets
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