How to add symbol with custom cursor input into a html textarea

I found this website and couldn’t figure out how this was possible. I am working on a project and needed something similar but found no way to do so. Here is an image:

The rendered area is a textarea. I checked the textcontent but it was empty. The textarea does not have the editablecontent tag.

How are they able to add custom images with boxes for the cursor to jump to and input with just a textarea. I was maybe thinking it was a font but I am not sure.

Here is a link to the page to try it out: https://www.imatheq.com/imatheq/com/imatheq/math-equation-editor.html.

I would greatly appreciate if anyone knows how this is done.