Mat-AutoComplete error when using formArray

I added a formarray to account for multiple rows. In order to make it work with the index I had to change my definition from : shoppingCartList Observable<any[]>; to shoppingCartList: Observable<any[]>[] = []; however when I do that it throws an error in my filter function saying missing the following properties from type 'Observable<any[]>[]': length, pop, push, concat. it looks like it's because I am calling a service to filter a list. What is the correct way to implement this: .HTML

<mat-autocomplete #auto="matAutocomplete" [displayWith]="entry">
       <mat-option *ngFor="let case of shoppingCartList[i] | async" [value]="case">
            
         </mat-option>
 </mat-autocomplete>

.TS

    shoppingCartList: Observable<any[]>[] = [];
            
             this.shoppingCartList[index] = arrayControl.at(index).get('name').valueChanges //ERROR: (property) DraftformComponent.filteredPrimaryCaseSerial: Observable<any[]>[]
    Type 'Subscription' is missing the following properties from type 'Observable<any[]>

                .pipe(debounceTime(this.debounceTime))
                  .subscribe(
                    (val) => {
                        this.filteredList= this.getFilteredlist(val); //ERROR: (property) DraftformComponent.filteredPrimaryCaseSerial: Observable<any[]>[]
    Type 'Observable<any[]>' is missing the following properties from type 'Observable<any[]>[]
                      }
                  );
        
         public getFilteredList(val: string): Observable<any[]> {
            return this.myService.getListByValue(val);
          }

from Mat-AutoComplete error when using formArray