In that repo, take a look in public/.htaccess This is where the magic happens. It's the web server which actually does the job of mapping the url to parameters. The code looks like this:
RewriteEngine on
RewriteBase /
RewriteRule ^category/(.*)$ index.php?page=category&cat=$1 [L]
What that's doing is saying any url that looks like category/ will be rewritten to index.php?page=category&cat=
So index.php actually handles all those requests, and has the correct GET value - even though the url used is different.