Moving everything from root to a subfolder

I am about to install a small (but very useful) "Coming Soon" set of pages available here https://prelauncher.net which, as far as I could understand after downloading it, has been created using CodeIgniter. If I manage to let it work as expected, such tool will be useful to gather some contacts before I open my small website.
All the system comes inside a /prelauncher/ folder, which is nice and tidy.
Now: if I take all the files and folders that I find in there, place them at root level, and type say https://example.com it all works fine.
If instead (as I would prefer to) I keep all the package inside its own /prelauncher/ folder and type something like https://example.com/prelauncher/ then the system does not seem to work as it should.
Where is the trick, please?
I read the manual, but it is far from clear (unless you already know how such things should work) and real examples are not available. 

In short: if the whole thing works when all files are placed at root level, what and how should be changed in order to let it run when all the files are placed inside a /subfolder/ please?

Thanks!