To get the closest element by a selector, you can use the element.closest(). See the following example:

const closestElement = targetElement.closest(selector);

<ul id="parentElement" class="level-1">
      <li id="ii" class="product-group">Web Development
        <ul class="level-2">
          <li class="child-1">HTML</li>
          <li class="child-2">CSS
            <ul class="level-3">
              <li class="css-1">test-1</li>
              <li class="css-2">test-2</li>
              <li class="css-3">test-3</li>
            </ul>
          </li>
          <li class="child-3">JAVASCRIPT</li>
        </ul>
      </li>
  </ul>

Below js code selects the closest <ul> element of the selected element and changes the background of the selected element to blue color :

<script>
    const x = document.querySelector("li.child-1");
    const closet = x.closest("ul");
    closet.style.background = 'blue';
</script>