What's the equivalent service for dmmonitor in CentOS 7?

we don't know what you are talking about. Googling on dmmonitor takes me to Windows documentation on data bases. There is no such tool in LInux AFAIK.

sudo systemctl [start/stop/restart/enable/disable] mdmonitor.service

It is now a systemd service in RHEL 8 and RHEL 7. RHEL 8 does not seem to have /etc/mdadm.conf.

To be honest I haven't used software raid for ages (the hardware raid is better as is mentioned in the course and have thought about eliminating this section.) We will have to go back and see what if anything needs modifcation in the course to make the lab work etc.

Next time please correct your typo faster as several of us went down the rabbit hole. Telling us exactly what section it referred to would have helped.

A little note on Chapter 24 / RAID and Ubuntu:

You mention that "on Ubuntu systems, the service is called mdadm rather than mdmonitor."

Here is what I get on a Ubuntu 18.04 system:

$ sudo systemctl start mdadm
Failed to start mdadm.service: Unit mdadm.service is masked.
$ sudo systemctl status mdadm
● mdadm.service
   Loaded: masked (/dev/null; bad)
   Active: inactive (dead)
 

However, mdmonitor works like a champ:

$ sudo systemctl start mdmonitor.service 
$ sudo systemctl status mdmonitor.service 
● mdmonitor.service - MD array monitor
   Loaded: loaded (/lib/systemd/system/mdmonitor.service; static; vendor preset:
   Active: active (running) since Tue 2020-10-06 12:34:25 IDT; 7s ago
 Main PID: 3518 (mdadm)
    Tasks: 1 (limit: 4915)
   CGroup: /system.slice/mdmonitor.service
           └─3518 /sbin/mdadm --monitor --scan
 
Oct 06 12:34:25 worker4 systemd[1]: Started MD array monitor.
 

It seems to execute mdadm --monitor --scan, though.

About deleting the chapter, the less we need to study, the better . But unfortunately in real life we could learn a tough lesson when implementing hardware RAID and not purchasing enough spare controllers. mdadm always works. That's the beauty of Linux.