On Red Hat distributions and derivatives, typically the package installer has a "create user" command inside the pre-install script. So, on CentOS 7, via the `rpm -q --scripts httpd` we can see

 preinstall scriptlet (using /bin/sh):
 # Add the "apache" group and user
 /usr/sbin/groupadd -g 48 -r apache 2> /dev/null || :
 /usr/sbin/useradd -c "Apache" -u 48 -g apache \
 -s /sbin/nologin -r -d /usr/share/httpd apache 2> /dev/null || :

This tells us a number of things:

* On Red Hat and derivatives the user is called `apache`
* It also tries to create a group called `apache`
* It wants the userid and groupid to be 48
* But if the user or group already exist then it continues with the install.