What is the difference between the config for "Horde" and how is Horde a "virtual web server"?
horde is a php application. it is not a service per se.
so to access it, you need to use a service to serve it. in fact 2.
first service httpd which use virtualhost configuration to define an dedicated web host for horde.
second service php-fpm to interprete the php code to allow httpd to serve the page to your client application (mostly firefox, safari, edge or chrome). it has also a dedicated php-fpm pool also called after horde.
and finally your horde app is in fact a collection of app with multiple configuration files.
you now know the very tip of the iceberg.