It looks like this ebook is done with bookdown:

https://mixtape.scunning.com/

When hovering over a reference or footnote, the information is displayed already:

What is the name of this feature?

>Solution :

The current online version of Causal Inference: The Mixtape is actually done using Quarto, not {bookdown}. And AFAIK, it seems Quarto makes use of Tippy.js to show footnotes on hover.

However, bookdown::bs4_book() has similar functionality. It has an argument footnotes_inline which is by-default TRUE.

footnotes_inline: By default, footnotes will be set inline and shown on hover. Set to FALSE to keep footnotes at the bottom of the page with links.

As an example of bookdown::bs4_book, checkout the footnote used in this section of Mastering Shiny book