How button and ul of dropdown are connected?

It's from the official documentation of Boostrap. I've deleted the aria-labelledby="dropdownMenu1" attribute from the ul element.

<div class="dropdown">
      <button class="btn btn-default dropdown-toggle" type="button" id="dropdownMenu1" data-toggle="dropdown" aria-haspopup="true" aria-expanded="true">
        Dropdown
        <span class="caret"></span>
      </button>
      <ul class="dropdown-menu">
        <li><a href="#">Action</a></li>
        <li><a href="#">Another action</a></li>
        <li><a href="#">Something else here</a></li>
        <li role="separator" class="divider"></li>
        <li><a href="#">Separated link</a></li>
      </ul>
</div>

How the button does know what to drop down?