Wondering what's this part is called in design. Also the string in the orange box is not included in the email content when you open the email. How this string is created? Generated automatically? Written by UX writers?
I think you're referring to the Pre header text
In brief, a preheader is a text that comes after the subject line when an email is viewed in your inbox. It’s often used to give your email list a taste of what’s inside the body of your email.
Here's an example of a hidden pre-header once you open the email from the article:
If the context is the UI/UX itself: It is called a "microcopy". And yes, you can learn about how to write it effectively by searching for the term "UX Writing". https://uxwritinghub.com/what-is-microcopy/
If the context is the real content of the email: it is called "lead paragraph" or just "lead". (https://en.wikipedia.org/wiki/Lead_paragraph) It comes from the journalism. It is a short summary of the following text.
I hope it helped. ;)