I've installed LAMPP stack that includes Apache, from Apache documentation I understand that if I install Apache form package manager, the default configuration file will be /etc/apache2/apache2.conf, but if I install it from something else it may be in /opt (which is the case here, as I install it with LAMPP stack which locate config file in /opt/lampp/etc/httpd.conf).

now which one is the default config file for Apache? both of them write in the beginning of the file that it's the default page.

DocumentRoot in /opt/lampp/etc/httpd.conf is set to /opt/lampp/htdocs and DocumentRoot in /etc/apache2/apache2.conf is www/var/html, but when I hit http://localhost/ in URL it takes me to /opt/lampp/htdocs. Is that make me sure that the /opt/lampp/etc/httpd.conf is the default one?

and why there are two default config file, why there are /etc/apache2 folder at all,

I see this link that said he install XAMPP, and he doesn't has apache2 folder, is that mean that I install Apache alone after installing LAMPP?(maybe I do it, I really doesn't remember), and is there any bad harm if I delete that apache2 folder?