JSFiddle - React, Tailwind, and code Playground

HTML

<header>
  <nav>
   <ul>
     <li>
       <a href="#">Unternehmen</a>
       <a href="#" class="submenubutton"><img src="images/icons/Arrow_open_S.svg" /></a>
       <ul>
         <li>
           test
         </li>
       </ul>
     </li>
   </ul>
 </nav>
</header>

CSS

header nav ul {
  position: relative;
  list-style-type: none;
  top: 51px;
}
header nav ul li {
  list-style-image: none;
  display: inline-block;
  margin-left: 30px;
  margin-right: 30px;
  height: 60px;
}
header nav ul li:hover > ul {
  position: absolute;
  top: 40px;
  display: block;
  margin: 0;
  padding: 0;
}
header nav ul li > a:first-child {
  font-size: 14px;
  font-weight: 700;
  letter-spacing: 0.15em;
  color: black;
  text-transform: uppercase;
}
header nav ul li > a:first-child:hover {
  text-decoration: none;
}
header nav ul li > a:first-child:hover + a:after {
  content: "";
  position: relative;
  display: block;
  height: 2px;
  width: 100%;
  background: black;
}
header nav ul li a.submenubutton:last-child {
  display: none;
}
header nav ul li > ul {
  position: absolute;
  left: 0;
  width: 100%;
  display: none;
  background: black;
  color: white;
}
header nav ul li > ul:hover {
  display: block;
}
header nav ul li > ul:hover:before {
  content: "";
  position: relative;
  display: block;
  height: 2px;
  width: 100%;
  background: black;
  bottom: 4px;
}
header nav ul li > ul li {
  list-style-image: none;
  display: inline-block;
  margin-left: 30px;
  margin-right: 30px;
}