We will create a new post with a comment. This is done in a tool call Gutenberg that WordPress has pre-installed for you with every fresh site. There are quite a few options available to you on how you want to structure your posts but I will show you a few.
Then, we'll look at how that post is rendered on WordPress's frontend and work out how we'd gather that same data on the REST API. You'll see in the end how you will need to make several API calls to fetch all the data you need through the API. This is because the initial call to
/posts/#ID gives you all the posts' contents but also gives you the ID to the author and comments on the post that you will need to fetch to display correctly.