I never use Wordpress but I usually use Xampp (Linux) to work.
So, I installed Wordpress and it works fine, if I enter with my browser to http://localhost/wordpress/ I can see the WP page.
I got from a friend his WordPress site files and stored them in /opt/lampp/htdocs like any regular web site, but when I try accessing http://localhost/folder_name/ I get the Index of folder_name and not the actual site.
I noticed that /opt/lampp/apps/ has a "WP" folder, and inside it - an htdocs folder like xampp have.
Where's the correct place to store the WP project ?
Is it /opt/lampp/apps/wp/htdocs or opt/lampp/htdocs/ ?
How can I get the WP site working and not get the index of / page?
You want to put the root of your WordPress installation in the same location you would put anything you want to be able to access at http://localhost. From what you're saying, it sounds like that is
/opt/lampp/htdocs/The Problem
WordPress runs just about everything through a database, including it's
siteurlandhomedirectory values. If you don't have a database at all, nothing is going to work. You'll find more on this problem Here.The Hard Fix
You could try Migrating your friend's WP install to your XAMPP server, but you'll need their database for that to work. You could then use a tool like Wordpress Database Reset, which will get you back to basically a fresh install, just with any themes and plug-ins your friend's install included.
The Easy Fix
Since it sounds like you want a fresh install with a custom theme or plug-in from your friend, you can just start with a fresh install, and then add any themes and plug-in's in. Installing WordPress is fairly simple since they have a web-based installer to help you get everything set up. Check out this guide. Once you've got the basic site set up you can import any themes and plug-ins you want from your friend's WordPress installation.
I hope this helps!