Font Awesome not working, icons showing as squares

According to the documentation (step 3), you need to modify the supplied CSS file to point to the font location on your site.

You must have 2 classes, the fa class and the class that identifies the desired icon fa-twitter, fa-search, etc …

<!-- Wrong -->
<i class="fa-search"></i>    

<!-- Correct -->
<i class="fa fa-search"></i>

Bootstrap 5 update

Note: "The fa prefix has been deprecated in version 5. The new default is the fas solid style and the fab style for brands." – Terje Solem