Use Email Template Builder Templates in Lightning Experience

Anyone who can send an email can insert a public email template when composing an email.

Required Editions

Available in: Lightning Experience for email templates created in Email Template Builder and Email Content Builder
Available in: Professional, Enterprise, Unlimited, Performance, and Developer Editions.
Available in: Account Engagement Lightning App for all Account Engagement editions
User Permissions Needed
To send email: Send Email permission and access to the record the email is sent from
  1. Start an email or list email.
  2. In the email composer, click the Template icon.
  3. Choose an Email Template Builder template.