Apache 2.4. Map subdomain to directory with mod_rewrite

_TLDR_no_comment_version.conf

  RewriteEngine On
	RewriteMap lc int:tolower

	RewriteCond %{HTTP_HOST} ^([^.]*)\.([^.]*)\.([^.]*)\.([^.]*)$
	RewriteRule ^.*$ - [R=404,L]

	RewriteCond %{REQUEST_URI} \/icons\/.*
	RewriteRule ^.*$ - [L]

	RewriteCond %{HTTP_HOST} ^([^.]*)\.([^.]*)\.([^.]*)$
	RewriteRule ^(.*)$ /var/www/${lc:%2}\.${lc:%3}/${lc:%1}/$1

explained.conf

  # This rewrite will map subdomain to directory
  #   For example:
  #     123.example.com  -> /var/www/example.com/123/
  #     test.example.com -> /var/www/example.com/test/
  #     www.example.com -> /var/www/example.com/www/
  # An extra Vhost for example.com -> /var/www/example.com/www/ will be required.
  
  # Enable Rewrite
  RewriteEngine On
  # Define a mapping to convert string to lowercase
	RewriteMap lc int:tolower

  # Optional. 404 for extra level of subdomain(a.b.example.com). 
	RewriteCond %{HTTP_HOST} ^([^.]*)\.([^.]*)\.([^.]*)\.([^.]*)$
	RewriteRule ^.*$ - [R=404,L]
	
  # Optional. If URI matches the alias defined, exclude it from mapping
  #   Example: Alias of /icons/ is defined in mods-available/autoindex.conf (by enabling mod_autoindex)
  #     If it's not excluded from mapping here, 404 will be returned.
  # Repeat this for each Alias defined.
	RewriteCond %{REQUEST_URI} \/icons\/.*
	RewriteRule ^.*$ - [L]

  # Here comes the real mapping
  # Explanation
  #   Split the host by '.'
  #   Convert the domain part to lowercase, and map to directory
  # Other Styles:
  #   123.example.com -> /var/www/com/example/123/
  #     RewriteRule ^(.*)$ /var/www/${lc:%3}/${lc:%2}/${lc:%1}/$1
  #   laravel.example.com -> /var/www/com/example/laravel/public/
  #     RewriteRule ^(.*)$ /var/www/${lc:%3}/${lc:%2}/${lc:%1}/public//$1
	RewriteCond %{HTTP_HOST} ^([^.]*)\.([^.]*)\.([^.]*)$
	RewriteRule ^(.*)$ /var/www/${lc:%2}\.${lc:%3}/${lc:%1}/$1
  
  # FAQ:
  # Why lowercase?
  #   Yes, your browser always convert domains to lowercase
  #     However, the value comes from HTTP Header "Host", which could be upper case while using other tools(Postman/Telnet..)
  # Securiy concerns?
  #     For the Host header, only [a-zA-Z-_\.] are possible. so the file system would be fine

You will need an additional "RewriteBase /" in your app's public directory. (public/.htaceess for Laravel as an example)