To put it clearly, just saying ? means sending no getdata to whatever the default page is at the current directory...
is functionally identical to saying:
basically if defaultpage was a variable...
where defaultpage could be any one of:
or whatever else the server it's running on is set up to check as the default page for /
In Apache the list of files to be called as the index is set via the directoryIndex variable, which you can change or customize with a .htaccess file... and these days typically starts out set to this:
DirectoryIndex index.html index.htm default.htm index.php index.php3 index.phtml index.php5 index.shtml
works like font-family does in CSS... if the first one is found, it's used, if not try the next one and lather-rinse repeat until one is found.