I would add the -f option to the OPTIONS variable in /etc/sysconfig/httpd. Of course, before you start the service with -f /app/apache/configfile, /app/apache/configfile must exist.

Simply copying /etc/httpd to /app/apache is not enough. You need to check if there are references to /etc/httpd in the current configuration and adjust them.

No need to remove /etc/httpd. I would keep it for an easy fallback option, just in case.

Do I make the changes to /etc/systemd/system/httpd.service? What exactly do I put in the httpd.service file? What does the $OPTIONS -f entry look like?

In the new /app/apache/conf/ directory I have an Include at the bottom of the httpd.conf file:

######
Include conf/dev-test.conf

Inside dev-test.conf I have all the requisite pointers to /app/apache. I also assume in the httpd.conf file there I update ServerRoot "/app/apache"?

I said:
You add "-f /app/apache/whateverthenameofyourconfigurationfileis" to the OPTIONS string. Unfortunately I don't have an example in front of me. A line like this probably works:
I think so, especially if the original value of ServerRoot is /etc/httpd.

So I don't update the httpd.service file in /etc/systemd/system/?

In /etc/sysconfig/httpd I've modified it to have:

#
OPTIONS=${OPTIONS} "-f /app/apache/conf/httpd.conf"

And the ServerRoot is updated in the /app/apache/conf/httpd.conf file as well.

Now if this is done correctly, do I run:

systemctl daemon-reload or do I use systemctl reload httpd.service?

And then start it up: systemctl start httpd.service

Yes.

Here's the error when I run it:

# systemctl start httpd
Job for httpd.service failed because the control process exited with error code. See "systemctl status httpd.service" and "journalctl -xe" for details.

# systemctl status httpd
● httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)
Active: failed (Result: exit-code) since Wed 2019-08-14 15:39:10 EDT; 23s ago
Docs: man:httpd(8)
man:apachectl(8)
Process: 12246 ExecStop=/bin/kill -WINCH ${MAINPID} (code=exited, status=1/FAILURE)
Process: 12239 ExecStart=/usr/sbin/httpd $OPTIONS -DFOREGROUND (code=exited, status=1/FAILURE)
Main PID: 12239 (code=exited, status=1/FAILURE)

Aug 14 15:39:10 dev-test.machine httpd[12239]: -M : a synonym for -t -D DUMP_MODULES
Aug 14 15:39:10 dev-test.machine httpd[12239]: -t : run syntax check for config files
Aug 14 15:39:10 dev-test.machine httpd[12239]: -T : start without DocumentRoot(s) check
Aug 14 15:39:10 dev-test.machine httpd[12239]: -X : debug mode (only one worker, do not detach)
Aug 14 15:39:10 dev-test.machine systemd[1]: httpd.service: main process exited, code=exited, status=1/FAILURE
Aug 14 15:39:10 dev-test.machine kill[12246]: kill: cannot find process ""
Aug 14 15:39:10 dev-test.machine systemd[1]: httpd.service: control process exited, code=exited status=1
Aug 14 15:39:10 dev-test.machine systemd[1]: Failed to start The Apache HTTP Server.
Aug 14 15:39:10 dev-test.machine systemd[1]: Unit httpd.service entered failed state.
Aug 14 15:39:10 dev-test.machine systemd[1]: httpd.service failed.

Does anyone have any idea how I can fix this issue? Any advice is welcome.

You could check the journal, as suggested. I would run journalctl -u httpd, then in the program enter minus followed by upper case S (i.e., -S) so that full lines are displayed.

However the error message indicates incorrect command line options. And indeed, something is wrong with OPTIONS:
The space before the first double quote character causes the command to fail. Stick the quote right to the OPTIONS and put the space after it:
Sorry I should have caught that earlier.

Made the changes, but still getting the same errors.