Solr does not appear to be running in CF 2018 install

Michael, you just need to start the "cfjetty" process.

You may have noticed that when you install CF on Linux, it ALSO does not start up automatically. You go to the cfusion/bin folder and run the coldfusion start command. Well, similarly, since CF10, you would go to the cfusion/jetty folder, and run the cfjetty start command. Once you do that, both the "collections" page and your call to the {server}:8991/solr  url should now work.

BTW, the "cfjetty" process covers both Solr and PDFG (for the CFHTMLTOPDF tag).

FWIW, if you want to be able to have it run all the time, automatically, you can copy that cfjetty file to /etc/init.d, as in sudo cp /opt/coldfusion2018/cfusion/jetty/cfjetty /etc/init.d

So no, you don't need to install it separately. There is indeed an available installer for that (which one can download from Adobe at the cf downloads page), but that's for when you want to install it on a server SEPARATE from the CF server. Since as you say you do see the cfusion.jetty folder, I am assuming that you or whoever installed CF did choose its option to install Solr. 

If you may try to start it, and find either that it doesn't start, or the URL you visit doesn't work, there are logs in the cfusion/jetty/logs folder which may help, and there are config files in the jetty and jetty/etc files that can be tweaked for some needs. But let's hear if the first point above is all you may need to get going.