Htaccess how to redirect to 503 with get check?

Question

htaccess

How to make a stub page with a check on the GET parameter access=y, and if it is, it will open the site?
For example, a user visits the page www.site.ru/company/, then he is shown a file with a stub (let's call it 503.html), but if the url contains $_GET access=y (www.site.ru/company/?access= y), then he will be shown a normal site with all the pages if you navigate through them?

Answer 1

this does not suck htaccess
all it can do is redirect to your CNC
Ideally, the logic should be like this
All requests are sent to a specific php file that already processes all codes and returns content.
How to make CNC
Other ways will not be as correct as we would like