Example of how to specify and access selectable as well currently selected option in LWC Combobox

parentComponent.js

export default class ParentComponent extends LightningElement {
  options = [
    {
      label: 'Option 1',
      value: 'option1'
    },
    {
      label: 'Option 2',
      value: 'option2'
    },
    {
      label: 'Option 3',
      value: 'option3'
    }
  ];

  selectedOptions = [];

  handleChange(event) {
    this.selectedOptions = event.detail;
  }
}