Drupal 9 Multisite setup

Here, Root site is http://www.drupal9.local/

Subsites or sites within root site are http://site1.drupal9.local/ and http://site2.drupal9.local/

Lets start with some easy steps:

1) To start the setup, first install Drupal 9 on a server with the database name 'drupal9'.

After installation create a virtual host for the root site as http://www.drupal9.local/

2) Create directory structure for multisites.
Create folders for site1 and site2 as:
/drupal9/sites/site1.drupal9.local
/drupal9/sites/site2.drupal9.local

3) Create databases for site1 and site2 as 'drupal9_site1' and 'drupal9_site2.

4) Let the root site aware about multisites. Create an $sites array in sites.php

Make a copy of /drupal9/sites/example.sites.php called /drupal9/sites/sites.php

5) Edit sites.php to add array. Add following array at the end of the file.

      $sites = [
                'site1.drupal9.local' =>'site1.drupal9.local', 
                'site2.drupal9.local'=>'site2.drupal9.local' 
               ]; 

6) Create virtual hosts for site1 and site2.

site1 : http://site1.drupal9.local/
site2 : http://site2.drupal9.local/

Note: Here virtual hosts for multisites should point to root site, not to sites subdirectories.

7) Now make settings for site1 and site2.

Copy /drupal9/sites/default/default.settings.php to the new site's directories as settings.php:
/sites/site1.drupal9.com/settings.php
/sites/site2.drupal9.com/settings.php

8) Now finish the installation for site1 and site2 by visiting the domains http://site1.drupal9.local/ and http://site2.drupal9.local/