Inserting row into table using SQLite on Android and I don't understand why there is no value for the primary key

I'm working on a project and don't understand this part of this code that I found online. (I have also looked at other examples and they do the exact same thing but I don't quite understand why)

When they are inserting something into the table, they have no value for the primary key. Could someone explain to me why that is the case?

Here is 2 examples of code that I found that do what I have stated above. Thanks.

//As you can see a contact has 3 atribbutes.
int _id;
String _name;
String _phone_number;


// Where they create a table. As you can see the primary key is ID
@Override
public void onCreate(SQLiteDatabase db)
{
   String CREATE_CONTACTS_TABLE = "CREATE TABLE " + TABLE_CONTACTS + "("
   + KEY_ID + " INTEGER PRIMARY KEY," + KEY_NAME + " TEXT, + KEY_PH_NO + " TEXT" + ")";

    db.execSQL(CREATE_CONTACTS_TABLE);
}


   // Adding new contact
   //*This is what I don't understand. Why don't they get an ID for the contact. 
    //They only have values for the name and phone number when they insert it into the table.* 
   public void addContact(Contact contact) 
   {
       SQLiteDatabase db = this.getWritableDatabase();

       ContentValues values = new ContentValues();
       values.put(KEY_NAME, contact.getName()); // Contact Name
       values.put(KEY_PH_NO, contact.getPhoneNumber()); // Contact Phone Number

        // Inserting Row
       db.insert(TABLE_CONTACTS, null, values);
       db.close(); // Closing database connection
   }

Heres another example but this is using a book. A book has 3 attributes, an id (the primary key), an author and the book name. And once again, they don't get the value for the primary key.

    public void addBook(Book book)
    {
         Log.d("addBook", book.toString());
         // 1. get reference to writable DB
         SQLiteDatabase db = this.getWritableDatabase();

         // 2. create ContentValues to add key "column"/value
         ContentValues values = new ContentValues();
         values.put(KEY_TITLE, book.getTitle()); // get title 
         values.put(KEY_AUTHOR, book.getAuthor()); // get author

         // 3. insert
         db.insert(TABLE_BOOKS, // table
            null, //nullColumnHack
            values); // key/value -> keys = column names/ values = column values

        // 4. close
        db.close(); 
      }