I would like to route a subdomain (https://test.mydomain.org) to https://mydomain.org/test. This without a redirect page. Is this possible with GRAV?
This should work by default, if you remove subdomain config. And to redirect I’d probably add a CNAME entry to domain DNS records
The DNS entry is for a physical destination. Where is this in the GRAV structure? Surely it is not …/home/www/grav/user/page/08.test? Where do I physically call up the Test page?
Based on your original post, your Grav installation is on a test folder. After your reply, I actually have no idea what your setup is 
Sorry, perhaps I have expressed myself incorrectly.
The CMS GRAV is installed in the folder “…/www/grav” and the domain mydomain.org is rooted to this folder.
Now I would like to root the subdomain test.mydomain.org (this does not have to be a subdomain, could also be another domain, e.g. test95.org) directly to the page https://mydomain.org/test’, which is not on the root of the CMS? I don’t want to have to create a redirect page on the root. Is that possible?
What does that mean? I’ve never seens such expression, that domain is rooted to something. Is there some htaccess rewrite? Is there a redirect?
As I said, I would just add a CNAME entry
test.mydomain.org. 1 IN CNAME mydomain.org/test.