Magento2 : Key Points

GROUPED PRODUCT 
Group of simple products. In this type, you cannot specify a specific price for product; you can just specify the discount CONFIGURABLE PRODUCTS In this type, customer can select products according to their color and size before purchasing. Example: Cell phones obtained in different colors and sizes.

BUNDLED PRODUCTS 
Bundled products are those products which cannot be sold separately and doesn’t give any choice for end user.

WHAT IS THE USE OF BACKORDER FIELD IN PRODUCT STOCK OPTION PANEL? 
If it is enabled, customer can buy products even if they are out of stock.

WHAT ARE THE LIMITATIONS OF MAGENTO 
Magento requires more space and memory. It can consume gigabytes of RAM during heavy processes. It becomes complex if it is not using object-oriented programming.

HOW CAN YOU ENHANCE THE MAGENTO PERFORMANCE? 
Disable the Magento log Disable any un-used modules Magento Caching Optimize your image Optimize your Server Use a Content Delivery Network (CDN) Put Stylesheets at the Top Put Scripts at the Bottom Avoid CSS Expressions

WHAT IS EAV IN MAGENTO? 
EAV stands for Entity Attribute Value. It is a technique that facilitates users to add unlimited columns to their table virtually.

Difference between Registry and Session in Magento :
 The Magento registry is not persisted, as in once you are on a new page you will not see those registry variables still set. I mainly use the registry to communicate between controllers and blocks.

public function __construct(
   \Magento\Framework\Registry $coreRegistry, 
   ) 
   {
        $this->coreRegistry = $coreRegistry; // declare Registry object

   }
$this->coreRegistry->register('variable_name', 'variable_value'); // set variable 
$this->_registry->registry('active_merchandisetab'); // Get variable

The session will persist, but know that there are multiple namespaces for sessions in Magento, and they will be cleared at certain times, like the checkout/session being cleared after the order is placed. It's best practice to create your own namespace for your session to avoid any conflicts like duplicate variables or clearing it at the wrong time.

Basic concepts of Magento models, resource models, and collections :
Magento Model :
Magento Model is handle all logical operation with database.Its ActiveRecord-like/one-object-one-table Model, and there’s also an Entity Attribute Value (EAV) Model. Each Model also gets a Model Collection.
Magento Resource Model
Magento’s resource simply connect with the database and performs the CRUD operations. Actually they manage the different reading/writing connections.Its another layer between model and database when you interacting with database.
Magento Collection Model:
Collection referees the collection of models.it contains many models.When we want results based on different model then we need to create collection or we can use existing collection.
It gives us a whole load of convenience methods, making it easier to manipulate lists of entities and reducing the need to write complex sql.

composer.json
Composer is a dependency management for PHP and in Magento 2.Using the Magento 2 composer, there is no requirement of storing all code of each component, and you only need to create composer.json file and declare the needed components in that file .
Path:
app/code/<vendor-name>/<module-name>/composer.json

Default Layout XML File Name FrontEnd
Home Page : cms_index_index.xml
Product Page : catalog_product_view.xml
CheckOut : checkout_index_index.xml
CartPage : checkout_cart_index.xml

ReferenceContainer and ReferenceBlock
Basically what's defined as <container name="..." /> can be accessed by referenceContainer. what's declared as <block .... /> can be referenced by referenceBlock