The site physically on sub.dmn.ru should open as dmn.ru/sub/ - what should I write in htaccess?

Question

htaccess

The main site is landing pages for receiving applications, advertising traffic goes to it. I will designate it: dmn.ru
I physically placed an additional site-catalog with parsing on a subdomain so that it does not affect the performance of the main one. I will designate it: sub.dmn.ru
I want to show users and search robots that the catalog site works in the folder of the main domain
dmn.ru/sub/ dmn.ru/sub/url remained in the address bar.
In many examples I found, they do the opposite: they put the site in a folder, and show it as a subdomain. There are options for redirecting to a subdomain. But I need to leave the URL of the folder in the address bar, and process the request on a subdomain.
Tell me, if this is possible, what should be in htaccess on dmn.ru and sub.dmn.ru?
If it is not possible with htaccess, what PHP script should be placed in the dmn.ru/sub/ folder?