<header>
  <button class="menu__button">Abrir</button>
  <nav class="menu__nav">
    <ul>
      <li><a href="#">Home</a></li>
      <li><a href="#">Produtos</a></li>
      <li><a href="#">Contatos</a></li>
    </ul>
  </nav>
</header>

/* PRÉ AQUECENDO */
* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
button {
  padding: 5px;
  background: transparent;
  border: solid 3px tomato;
  border-radius: .5rem;
  font-weight: bold;
  width: 4rem;
  cursor: pointer;
}
button:hover {
  background: #000D;
  color: white;
}

ul {
  display: inline-flex;
  flex-direction: column;
  align-items: stretch;
  gap: 1rem;
  border: solid 2px black;
  border-radius: 5px;
}

li {
  list-style: none;
  padding: .3rem 1rem;
}

li:hover {
  background: #0003;
}

li:hover a {
    text-decoration: underline;  
}

a {
  text-decoration: none;
  color: black;
}