Why do we have to indent comment @1:38? It seems it would be nested within the query without indentation.

As long as there are the proper number of opening and closing brackets/braces, the code doesn't care how the CSS is laid out. It sees both of the following as the same:

@media screen and (max-width: 768px) {
  li {
    border: 1px solid black;
  }
}

@media screen and (max-width: 768px) {li{border: 1px solid black;}}

The reason we indent things is to make it easier for our human eyes to be able to more easily keep track of and maintain code.

In the case of comments, they actually can be put anywhere, but it's better to put your comment with the piece of code it is explaining. So, if you are explaining what the following if/then statement is doing, it is best to have the comment indented to the same level so you and others know what the comment is about.