Getting the product SKU in the header mini-cart

As far as I know, the header minicart will get the data from customer data

vendor/magento/module-checkout/view/frontend/web/js/view/minicart.js

define([
    'uiComponent',
    'Magento_Customer/js/customer-data',
    'jquery',
    'ko',
    'sidebar'
], function (Component, customerData, $, ko) {
    'use strict';
    ......
    this.cart = customerData.get('cart');
    ......
}

Look into the customer data js vendor/magento/module-customer/view/frontend/web/js/customer-data.js, we can get the customer data from local storage. For example, in your browser console, run the line: localStorage.getItem('mage-cache-storage'), we also can get the cart information.

{
  "cart": {
    "summary_count": 1,
    ....
    "items": [
      {
      ......   
        "qty": 1,
        "item_id": "11728",
        "configure_url": "http://magento2-demo/checkout/cart/configure/id/11728/product_id/1817/",
        "is_visible_in_site_visibility": true,
        "product_name": "Breathe-Easy Tank",
        "product_url": "http://magento2-demo/breathe-easy-tank.html",
        "product_has_url": true,
        "canApplyMsrp": false
      }
    ],
    .......
  }
}

Navigate to vendor/magento/module-checkout/CustomerData/DefaultItem.php

protected function doGetItemData()
    {
       .......
        return [
            'options' => $this->getOptionList(),
            'qty' => $this->item->getQty() * 1,
            'item_id' => $this->item->getId(),
            'configure_url' => $this->getConfigureUrl(),
            'is_visible_in_site_visibility' => $this->item->getProduct()->isVisibleInSiteVisibility(),
            'product_name' => $this->item->getProduct()->getName(),
            'product_url' => $this->getProductUrl(),
            'product_has_url' => $this->hasProductUrl(),
           .....
    }

vendor/magento/module-checkout/CustomerData/AbstractItem.php

/**
 * {@inheritdoc}
 */
public function getItemData(Item $item)
{
    $this->item = $item;
    return \array_merge(
        ['product_type' => $item->getProductType()],
        $this->doGetItemData()
    );
}

To get the SKU item, I think we need to add data to getItemData() (Should try with Plugin). And then override the template html vendor/magento/module-checkout/view/frontend/web/template/minicart/item/default.html

 <div class="product-item-details">

                    <!-- ko text: product_sku --><!-- /ko -->

Update Magento 2.1.0 version

In the Magento 2.1.0, you only need to override default.html. This is because the method doGetItemData has already the product sku.

Firstly, very good explanation from @Khoa TruongDinh about the flow of getting item in minicart template.

how can I change them around to add or remove product details?

I found a way how you can extend minicart template with custom attributes of the product. To do that first you need to override vendor/magento/module-checkout/CustomerData/DefaultItem.php with DI Preferences

Create app/code/Vendor/Module/etc/di.xml ot override DefaultItem object

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Magento\Checkout\CustomerData\DefaultItem" type="Vendor\Module\Preferences\MiniCartItem" />
</config>

Then create new object to override doGetItemData() method and add custom_attribute with key product_custom_attribute

File: app/code/Vendor/Module/Preferences/MiniCartItem.php

namespace Vendor\Module\Preferences;

class MiniCartItem extends \Magento\Checkout\CustomerData\DefaultItem
{

    public function __construct(
        \Magento\Catalog\Helper\Image $imageHelper,
        \Magento\Msrp\Helper\Data $msrpHelper,
        \Magento\Framework\UrlInterface $urlBuilder,
        \Magento\Catalog\Helper\Product\ConfigurationPool $configurationPool,
        \Magento\Checkout\Helper\Data $checkoutHelper,
        \Magento\Catalog\Helper\Output $helper,
        \Magento\Catalog\Model\Product $productModel
    ) {
        $this->configurationPool = $configurationPool;
        $this->imageHelper = $imageHelper;
        $this->msrpHelper = $msrpHelper;
        $this->urlBuilder = $urlBuilder;
        $this->checkoutHelper = $checkoutHelper;
        $this->helper = $helper;
        $this->productModel = $productModel;
    }

    /**
     * {@inheritdoc}
     */
    protected function doGetItemData()
    {
        $imageHelper = $this->imageHelper->init($this->getProductForThumbnail(), 'mini_cart_product_thumbnail');
        $product = $this->productModel->load($this->item->getProduct()->getId());
        return [
            'options' => $this->getOptionList(),
            'qty' => $this->item->getQty() * 1,
            'item_id' => $this->item->getId(),
            'configure_url' => $this->getConfigureUrl(),
            'is_visible_in_site_visibility' => $this->item->getProduct()->isVisibleInSiteVisibility(),
            'product_name' => $this->item->getProduct()->getName(),
            'product_url' => $this->getProductUrl(),
            'product_has_url' => $this->hasProductUrl(),
            'product_price' => $this->checkoutHelper->formatPrice($this->item->getCalculationPrice()),
            'product_image' => [
                'src' => $imageHelper->getUrl(),
                'alt' => $imageHelper->getLabel(),
                'width' => $imageHelper->getWidth(),
                'height' => $imageHelper->getHeight(),
            ],
            'product_custom_attribute' => $this->helper->productAttribute($product, $product->getCustomAttribute(), 'custom_attribute'),
            'canApplyMsrp' => $this->msrpHelper->isShowBeforeOrderConfirm($this->item->getProduct())
                && $this->msrpHelper->isMinimalPriceLessMsrp($this->item->getProduct()),
        ];
    }
}

Notice that I am injecting

\Magento\Catalog\Model\Product $productModel

to the construct method because I need to load full product data to access my custom_attribute. If there is a better way please tell me.

And finally you can display the new attribute in

view/frontend/web/template/minicart/item/default.html:

 <div class="product-item-details">

                    <!-- ko text: product_custom_attribute --><!-- /ko -->