How to correctly implement a subdomain directory in .htaccess?

Question

htaccess

There is such an entry in .htaccess:

# /admin/-> admin.site.com
RewriteRule ^admin - [L]
RewriteCond %{HTTP_HOST} (www.)?admin.site.com [NC]
RewriteRule (.*) admin/$1 [L]

The subdomain itself has connected and is accessible via admin.site.com, everything opens, but... I create another test

folder in the admin folder and the index.php file in it ; I go to admin.site.com/test (without putting a slash at the end) and it throws me to admin.site.com/admin/test/ . If you go to admin.site.com/test/ (this time putting a slash at the end) - there will be no redirect to admin.site.com/admin/test/ at all. I tried with crutches in php, like with if ($_SERVER['REQUEST_URI'] == '/admin/')... and there is an error 404. I added redirects with a slash at the end to the .htaccess folder admin :

RewriteEngine On
RewriteCond %{REQUEST_URI} !\&
RewriteCond %{REQUEST_URI} !\=
RewriteCond %{REQUEST_URI} !\.
RewriteCond %{REQUEST_URI} !\/$
RewriteRule ^(.*[^\/])$ /$1/ [R=301,L]

Everything works, but it's very crooked. For example, I can enter admin/ through the main domain and I will get a 404 error, or I can enter admin/test/ and I will be transferred to test/ .

What is the problem and how to fix it?