Hello
Yes, you may find constructions like __[some text]__ in HTML template files. They are template keys which are parsed with PHP code during building of a page. Each page has its own template keys and they are available and will be parsed on this exact page only.
But there is a number of global template keys like <bx_url_root />, <bx_text:_language_key />, etc and key listed in parseSystemKey method of BxDolTemplate class. You may find all of them in inc/classes/BxDolTemplate.php file. However there is no a key for current page URL.