Automating your emails to offload repetitive tasks is a great way to enhance your productivity. There are many online resources which help you achieve this by using modified copies of successful emails and refactor them in a way which helps you automate sending.
To ensure emails are 100% unique, online email template generators leverage various techs in the background which allows them to randomly produce variations in how an email is written to ensure what you are sending out is totally unique. How each service provider does this is generally not public information but a common trend nowadays is to use Artificial Intelligence.
Once you have a great email to work with you will leverage your email client's templating capabilities to further automate sending them out or using them as a reply. The applicable strategy herein heavily depends on the email client of your chosing (Gmail, Outlook, Mailbird, etc). Or you could also bind the templates to a keybind and use it on demand far more than just for email but also on Instant Messaging and other types of coloqual engagements.