Change favicon on Modern SharePoint sites
Favicons are associated to a website and are used to easily identify it from the address bar or bookmarks.
Changing the SharePoint default favicon on classic SharePoint usually involves a modification of the master page as explained here, this option is not available on modern sites but it’s still possible to modify it using an Application Customizer.
In this article you will find detailed instructions on how to create the application customizer as well as the final solution ready to be installed on your system.