What is the name of the approach to working with the API in which you get html tags in json format?

Question

API

Good day! I read 2 months ago, in an article that talked about the reasons for the transition of Medusa to Flutter, that they use a method in which they receive ready-made blocks that they insert on the page.
The question is, what is this "method" called? Is there any information on how to use it?