I've installed apache2 on a server running Ubuntu 20.04.1 LTS and it's current status is running. Shouldn't I be able to see that service also in the
/etc/services is not a list of services currently running on your computer. It is not even a list of possible services on your or on any particular computer: not all services are listed in
/etc/services, and implementations of some of the services listed in it might not even exist in your Linux distribution.
Instead, it is a mapping between port numbers and human-readable port names, for library routines like
getservbyport(3). Basically, it's a copy of (some version of) IANA's master list of service names and port numbers, with possibly some distribution-specific additions, using a specific format that's directly usable by those library routines.
For example, if you run
netstat -a -A inet, you may see some ports (typically the more well-known ones) identified by name, and the rest by number. If you add the
-n option, the port-number-to-name mapping will be omitted and you will see all the port numbers as just numbers.
One of the very few places where it is/was the norm to use port names exclusively is the
/etc/inetd.conf file for the classic
inetd daemon. In modern software, you are either expected to just use port numbers, or can opt to use either names or numbers as you wish.