Directives :

Directive Selector Description
NgbDropdown [ngbDropdown] Transforms a node into a dropdown.
NgbDropdownToggle [ngbDropdownToggle] Allows the dropdown to be toggled via click. This directive is optional.

Refer following links for usage:

Type URL
NG-Bootstrap Page https://ng-bootstrap.github.io/#/components/dropdown
Bootstrap Page https://getbootstrap.com/docs/4.0/components/dropdowns/
Template Page https://demos.pixinvent.com/apex-angular-admin-template/demo-1/components/dropdowns