FormStructure.ts

 this._FormGroup = this.fb.group({
      formName: '',
      decs: '',
      questions: this.fb.array([this.fb.group({ // First level array here. After bracet again using FormGroup function for creating AbstractControl.
        question: '',
        questionType: '',
        answers: this.fb.array([ // Second level array here. After bracet again using FormGroup function for creating AbstractControl.
          this.fb.group({ value: ''})
        ])
      })])
    });
    });