Why Account and Contact objects internally have Master-Detail relation even though they were in Lookup relation. Can anyone explain why?

I am new in Integration please help me to "write an Apex class to import data from https://jsonplaceholder.typicode.com/users and store it in a created custom object" and please tell me how can I achieve this and what process should I follow to achieve the requirement.

Is their is any difference between one to many or many to one relationship and what is the difference between owner and created by??

I am new in Integration please help me to "write an Apex class to import data from https://jsonplaceholder.typicode.com/users and store it in a created custom object" and please tell me how can I achieve this and what process should I follow to achieve the requirement.

Why Account and Contact objects internally have Master-Detail relation even though they were in Lookup relation. Can anyone explain why?

There was an unexpected error while verifying this challenge. Usually this is due to some pre-existing configuration or code in the challenge Org. We recommend using a new Developer Edition (DE) to check this challenge. If you're using a new DE and seeing this error, please post to the developer forums and reference error id: FHTZRFEY

Is their is any difference between one to many or many to one relationship and what is the difference between owner and created by??

Challenge - Create a form to enter new items and display the list of items entered. To make our camping list look more appealing, change the campingHeader component to use the SLDS. Similar to the unit, style the Camping List H1 inside the slds-page-header. Modify the campingList component to contain an input form and an iteration of campingListItem components for displaying the items entered.
The component requires an attribute named items with the type of an array of camping item custom objects.
The component requires an attribute named newItem of type Camping_Item__c with default quantity and price values of 0.
The component displays the Name, Quantity, Price, and Packed form fields with the appropriate input component types and values from the newItem attribute.
The JavaScript controller checks to ensure that the Name, Quantity and Price values submitted are not null.
If the form is valid, the JavaScript controller pushes the newItem onto the array of existing items, triggers the notification that the items value provider has changed, and resets the newItem value provider with a blank sObjectType of Camping_Item__c.


My answer - 
<aura:component >
<aura:attribute name="items" type="Camping_Item__c[]"/>
<aura:attribute name="newitem" type="Camping_Item__c[]"  default="{ 'sobjectType': 'Camping_Item__c',
                   'Quantity__c'=0, 'Price__c'=0}"/>
 <p>Name:
        <ui:inputText value="{!v.newitem.name}"/>
    </p>    
  <p>Packed:
        <ui:inputCheckbox value="{!v.newitem.Packed__c}"/>
     
    </p>    
  <p>Price:
        <ui:inputCurrency value="{!v.newitem.Price__c}"/>
    </p>
    <p>Quantity:
        <ui:inputNumber value="{!v.newitem.Quantity__c}"/>
    </p>
</aura:component>


Error -

Challenge Not yet complete... here's what's wrong: 
The campingList component isn't iterating the array of 'items' and creating 'campingListItem' components.

Please share the correct solution.