Scenario:

1. main website - web.site
2. subdomain - sub.web.site
3. another domain - website.com

1. about website.com
   we have to remove actual folder for this domain, presented in our server system
   rm -rf website.com {it will delete entire folder}
   
   ln -s web.site website.com {to point out WEB.SITE as target for finding drupal files}
   
    
2. making deal with route folder for drupal 8 installation files {web.site/public_html}
   ln -s ./ sub {considering, we use subdomain SUB.web.site}
   [syntax for symlinks:ln -s source_file myfile]
   
    
3. path: public_html/sites
   mkdir sub.web.site website.com


cp example.sites.php sites.php
    
4. nano sites.php {at the bottom}
   1)* URL: http://website.com
     $sites['website.com'] = 'website.com';
   * URL: http://sub.web.site
     $sites['sub.web.site'] = 'sub.web.site'; {also we can create folders with different names like simply 'SUB', but for not being confused it is better to leave the same full path in folder}
   
   either
   2)
   

   $sites = array(
      'web.site' => 'default',
      'website.com' => 'website.com',
      'sub.web.site' => 'sub.web.site',
   );

   
    - difference between drupal 7 and 8 also is - if you are not adept of drupal system, you have no option to direct drupal-system for setting up your installation
   Drupal_7: /admin/config/media/file-system -> gives an option for PUBLIC & PRIVATE paths
   Drupal_8: assumes, this option - to be modified only through settings.php
   during setup this folder is filled in.
    
5. Copying SETTINGS & SERVICES files
   
   cp default/default.settings.php website.com/settings.php

   cp default/default.settings.php sub.web.site/settings.php

    

   cp default/default.services.yml website.com/services.yml

   cp default/default.services.yml sub.web.site/services.yml
   
   
   
   
    
6. other settings according to drupal.org
   

   Multisite structure:

   -- core
   
   -- modules
      -- contrib
      -- custom
   
   -- themes
      -- contrib
      -- custom
   
   -- sites
      -- site1
            -- modules
               -- custom
               -- contrib
            -- themes
               -- custom
               -- contrib
            -- files
      -- site2
            -- modules
               -- custom
               -- contrib
            -- themes
               -- custom
               -- contrib
           -- files
      -- site3
            -- modules
               -- custom
               -- contrib
            -- themes
               -- custom
               -- contrib
           -- files
   
   

by virtue of: 1 - https://mediatemple.net/community/products/grid/204643210/how-do-i-use-s...
 2- https://www.ostraining.com/blog/drupal/drupal-multi-sites/
3 - http://www.kalose.net/oss/drupal-8-setting-multi-site/

USEFUL INFORMATION:

1. Drupal 7 multisite - https://www.drupal.org/docs/7/multisite
2. Drupal 8 multisite - https://www.drupal.org/docs/8/multisite

Enable per-site modules

In some cases, you may want one of your sites within your multisite to have its own modules. To enable this, you simply need to create the appropriate folders within the target site’s folder. In this example, we will enable site1 to have its own modules:

1. Create a ‘modules’ folder in site1’s subdirectory: /d8multisite/sites/site1.d8multisite.com/modules
2. Give apache write access to this folder with chown www-data /d8multisite/sites/site1.d8multisite.com/modules
3. Test it out:
   1. Move out of site1’s subdirectory (i.e. into the root site) and install the pathauto module with drush drush dl pathauto
   2. Move into site1’s subdirectory and install the ds module with drush drush dl ds
   3. Visit both sites and confirm:
      1. Pathauto is available in both sites
      2. Display Suite is only available in site1

Installing virtual hosts for Drupal sites and subsites

<Directory /home/www/drupal5/>
Options +FollowSymLinks Indexes
AllowOverride All
order allow,deny
allow from all
</Directory>