2
public function __construct(
    \Magento\Catalog\Helper\Category $categoryHelper,
    ....

Let's say we have code like that. How does Magento know to inject category helper and lets us use it? It works like total magic.

0

The new object is created in lib/internal/Magento/Framework/ObjectManager/Factory/AbstractFactory.php:93

protected function createObject($type, $args)
{
    return new $type(...array_values($args));
}

You can see how these arguments are created and cached in lib/internal/Magento/Framework/ObjectManager/Factory/Compiled.php:88

How are the constructor arguments read?

During the bootstrap initialization /var/di/{area}.ser files are loaded. These are previously created by the setup:di:compile command and they are basically serialized strings that contain argument relations.

I hope it answers your question to some extent :)

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