You're seeing just the function render, go back to Legendary.CoreEmail module for more information.
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render(email, template, assigns \\ [])

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Render an Phoenix template and set the body on the email.

Pass an atom as the template name (:welcome_email) to render HTML and plain text emails. Use a string if you only want to render one type, e.g. "welcome_email.text" or "welcome_email.html". Scroll to the top for more examples.