Autoload psr-4 in composer.json

Hello everyone,

I'd like to ask a few questions about autoload in composer.json. I was looking into this tutorial on publishing my own package to packagist... and got confused with what autoload in composer.json does.

https://pusher.com/tutorials/publish-laravel-packagist

Here are my questions...

A. What does autoload (espcially psr-4) in Laravel do in the background?

B. Why should each package have its own autoload psr-4? What is this for? ex)

 "autoload": {
            "psr-4": {
                "MyVendor\contactform\": "src/"
            }
        }

C. Why do we add another autoload psr-4 to Laravel's composer.json? What is this for? ex)

"autoload": {
            "classmap": [
                "database/seeds",
                "database/factories"
            ],
            "psr-4": {
                "MyVendor\Contactform\": "packages/MyVendor/contactform/src",
                "App\": "app/"
            }
        }

D. What's the relationship between this and the service provider list in config/app.php? It looks like I still need to add the custom package's service provider to app.php anyway. Then, what's the point of doing 'autoload'?

// config/app.php
        'providers' => [
         ...,
            App\Providers\RouteServiceProvider::class,
            // Our new package class
            MyVendor\Contactform\ContactFormServiceProvider::class,
        ],

Please enlighten me...

Thanks!