How do I utilize interface concept in a program and satisfy its concept?

You don't have to call all members of an interface, just call the ones you need. Reading between the lines, though, if your interfaces are effectively the same;

public interface Interfaces
   {
       void get();
       bool insert(User U);
   }

   public interface Interface2
   {
       void get2();
       bool insert2(User U);
   }

then you don't need Interface2 at all, simply have the class that implements Interface2 implement Interfaces instead.

Any Class (or, in C#, Struct) that implements an Interface must implement all Properties and Methods (and Indexers, and Events) specified in the Interface (C# interfaces do not allow Fields).

Keep in mind that a C# Interface is not a Class, or, a value, or reference, Type. While you can call some methods of 'Object on an Interface, like 'ToString(), the "nature" of an Interface is rather unique in .NET: see these comments by Jon Skeet: [^].

An Interface can inherit from another Interface, and I suggest you look at this as being an example of "extension;" and, you can use generic Type declaration(s) with an Interface:

public interface IController
{
    // property
    string Name { set; get; }
}

public interface IControllerItems<T> : IController
{
    // property
    List<T> Items { set; get; }
    
    // method
    int ModelNumber { set; get; }
    
    // method
    void AddItem(T item);
}

So, an Interface is a contract, a promise that you will implement, a demand that you must implement; I suggest you think of that as different from "inheritance" of Classes.

Once you have implemented an Interface in a Class, to what extent you make use of every Property and Method you've implemented is ... up to you.

Problem here. To be continued ...