You can either have a function to remove the filter like so:

resetFilter() {
    this.selectedBrand = '';
    this.filteredCustomer = this.customerTransferts
}

You could call this with a button click or whatever you like.

Or you can put another option in your dropdown, and check for it in the onChangeType method.

<select class="form-select" style="max-width: 100px" [ngModel]="selectedBrand" (ngModelChange)="onChangeType($event)">
    <option [value]="'ALL'">ALL</option>
    <option [value]="'IN'">IN</option>
    <option [value]="'OUT'">OUT</option>
</select>

public onChangeType(type: any) {
    if (type === 'ALL') {
      this.filteredCustomer = this.customerTransferts;
      return;
    }

    this.selectedBrand = type;
    console.log(this.selectedBrand);

    this.filteredCustomer = this.customerTransferts.filter(
        (item) => item.type === this.selectedBrand
    );
    console.log(this.filteredCustomer);
  }

Also you should change your ngModel to two way binding [(ngModel)]

<select class="form-select" style="max-width: 100px" [(ngModel)]="selectedBrand" (ngModelChange)="onChangeType($event)">

Then it will automatically be synced with the selectedBrand property - both ways.

So you can remove this.selectedBrand = type from onChangeType().

public onChangeType(type: any) {
    if (type === 'ALL') {
      this.filteredCustomer = this.customerTransferts;
      return;
    }

    this.filteredCustomer = this.customerTransferts.filter(
        (item) => item.type === type
    );
  }