How can i use DB Connection of another site of my multisite?
You can initialize multiple database connections like this in your settings.php file:

&lt;?php
$databases['default']['default'] = array(
    'database' =&gt; 'sitedb1',
    'username' =&gt; 'admin',
    'password' =&gt; 'admin',
    'prefix' =&gt; '',
    'host' =&gt; 'localhost',
    'port' =&gt; '3306',
    'namespace' =&gt; 'Drupal\\Core\\Database\\Driver\\mysql',
    'driver' =&gt; 'mysql',
);

$databases['yellow-site']['default'] = array(
    'database' =&gt; 'sitedb2',
    'username' =&gt; 'admin',
    'password' =&gt; 'admin',
    'prefix' =&gt; '',
    'host' =&gt; 'localhost',
    'port' =&gt; '3306',
    'namespace' =&gt; 'Drupal\\Core\\Database\\Driver\\mysql',
    'driver' =&gt; 'mysql',
);

$databases['green-site']['default'] = array(
    'database' =&gt; 'sitedb3',
    'username' =&gt; 'admin',
    'password' =&gt; 'admin',
    'prefix' =&gt; '',
    'host' =&gt; 'localhost',
    'port' =&gt; '3306',
    'namespace' =&gt; 'Drupal\\Core\\Database\\Driver\\mysql',
    'driver' =&gt; 'mysql',
);


Then you can get the connections with your module etc.:

&lt;?php
$greenSiteDatabase = Database::getConnection('default', 'green-site');
$yellowSiteDatabase = Database::getConnection('default', 'yellow-site');
$defaultDatabase = Database::getConnection('default', 'default');


You can query a database like so:

$banana = $yellowSiteDatabase-&gt;select('SomeTable', 'st')
  -&gt;fields('st', ['fruit'])
  -&gt;condition('fruit', $someCode)
  -&gt;execute()
  -&gt;fetchObject();