From the catalog, to add a site to your favorites, you must click on the little star on the left of its domain name. When the star is yellow, the site is in your favorites.
To find them, just click on my favorite sites in the left column. To remove a site from your favorites, click on the star again and refresh your page : the site no longer appears.
A site was in your favorites and is no longer : I can't find a site that was on the platform before.