How dependency injection works for Interfaces in constructors

Question

As far as I know you can set a preference for an Interface in the di.xml of an extension. But i see several Interfaces without any reference in the di.xml. How are these instantiated?

Examples: app/code/Magento/Directory/Model/ResourceModel/Country/Collection.php

The constructor contains that code:

 \Magento\Framework\DB\Adapter\AdapterInterface $connection = null,

or app/code/Magento/Payment/Model/Info.php

\Magento\Framework\Encryption\EncryptorInterface $encryptor

Answer 1

the preference for \Magento\Framework\Encryption\EncryptorInterface is set int app/etc/di.xml. That file is not module dependent but it is merged with the other di.xml files.
The $connection parameter defaults to null. This means that if it's not specified, the DI container will not try to instantiate the preference for \Magento\Framework\DB\Adapter\AdapterInterface. Instead it will just use null for this.