You will use HTML to establish the structure of your page. Most modern frameworks wrap HTML in something like JSX, which is just nice to use HTML.
HTML is a specification that tells a computer what certain data means. For example, if a browser sees
It knows that the tag
<img /> is telling the browser that
img_girl.jpg is a JPEG image and it should render the image as such.
When a browser receives HTML from a server, it will first read the text and construct a tree of all the elements specified in the HTML file.
The following HTML will be interpreted by the browser
And the browser will construct a tree of the elements
If you right-click and click inspect on this screen
You will see the DOM for this page
Then you click the element selector and see the different containers (nodes) that make up the screen.