All requests from m.site.ru/* to site.ru/* except m.site.ru (m.site.ru/index.php) - how?

Question

Apache HTTP Server

I know that this is a banal question, but I can’t figure out for myself what to write in .htaccess:
so that all requests to the address:
m.site.ru (including m.site.ru/index.php) get unchanged to a real file on the subdomain: m.site.ru/index.php
and all other requests ( m.site.ru/*****) went to the main domain site.ru
Thank you!