I remained generic in my post because I believe the topic is more general than this installation of Mint 19.3/Ubuntu 18.04 I use. Mint 20 behaves the same.
Moving on, there isn't a "/etc/rc.local" file in this installation. There are, however, folders named /etc/rc0.d thru /etc/rc6.d plus /etc/rcS.d, none of which contain rc.local. Likewise, there is no /etc/init.d/lampp script anywhere I looked on this computer.
I performed the edit described at the site I provided and it does exactly what I had to search for: Autostart XAMPP on system boot using Mint 19.3 (and/or Ubuntu. )
I have never even hinted I possess any Linux guru-ness. When I find an answer to a problem that has plagued me and Lord knows how many others, I share it without trying to be a snob about it. It's how we old guys roll.