One option is to make it a self-invoking function:
<script>
  (function() {
    alert("hola mundo");
  })();
</script>
More info: What is the purpose of a self executing function in javascript?
<!DOCTYPE html>
<html lang="es">
<head>
  <title>Hotel</title>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
  <link rel="stylesheet" href="hotel.css">
  <!-- CSS only -->
  <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.0/css/bootstrap.min.css" integrity="sha384-9aIt2nRpC12Uk9gS9baDl411NQApFmC26EwAOH8WgZl5MYYxFfc+NcPb1dKGj7Sk" crossorigin="anonymous"> **
  <script type="text/javascript" src="hola.js"></script>**
</head>
<body>
  <!-- agregar el carrusel !-->
  <div id="slider" class="carousel slide" data-ride="carousel">
    <ol class="carousel-indicators">
      <li data-target="#slider" data-slide-to="0" class="active"></li>
      <li data-target="#slider" data-slide-to="1"></li>
      <li data-target="#slider" data-slide-to="2"></li>
    </ol>
    <div class="carousel-inner">
      <div class="carousel-item active">
        <img src="aereo.jpg" class="d-block w-100" alt="vista aerea del hotel">
        <div class="carousel-caption d-none d-md-block">
          <h1 class="carusel_titulo" id="appear"> First slide label</h1>
          <p>Nulla vitae elit libero, a pharetra augue mollis interdum.</p>
        </div>
      </div>
      <div class="carousel-item">
        <img src="edificio.jpg" class="d-block w-100" alt="fachada del hotel">
        <div class="carousel-caption d-none d-md-block">
          <h1 class="carusel_titulo" id="appear">Second slide label</h1>
          <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</p>
        </div>
      </div>
      <div class="carousel-item">
        <img src="spa.jpg" class="d-block w-100" alt="spa del hotel">
        <div class="carousel-caption d-none d-md-block">
          <h1 class="carusel_titulo" id="appear">Third slide label</h1>
          <p>Praesent commodo cursus magna, vel scelerisque nisl consectetur.</p>
        </div>
      </div>
    </div>
    <a class="carousel-control-prev" href="#slider" role="button" data-slide="prev">
      <span class="carousel-control-prev-icon" aria-hidden="true"></span>
      <span class="sr-only">Previous</span>
    </a>
    <a class="carousel-control-next" href="#slider" role="button" data-slide="next">
      <span class="carousel-control-next-icon" aria-hidden="true"></span>
      <span class="sr-only">Next</span>
    </a>
  </div>
  <!-- JS, Popper.js, and jQuery -->
  <script src="https://code.jquery.com/jquery-3.5.1.slim.min.js" integrity="sha384-DfXdz2htPH0lsSSs5nCTpuj/zy4C+OGpamoFVy38MVBnE+IbbVYUew+OrCXaRkfj" crossorigin="anonymous"></script>
  <script src="https://cdn.jsdelivr.net/npm/popper.js@1.16.0/dist/umd/popper.min.js" integrity="sha384-Q6E9RHvbIyZFJoft+2mJbHaEWldlvI9IOYy5n3zV9zzTtmI3UksdQRVvoxMfooAo" crossorigin="anonymous"></script>
  <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.5.0/js/bootstrap.min.js" integrity="sha384-OgVRvuATP1z7JjHLkuOU7Xw704+h835Lr+6QL9UvYjZE3Ipu6Tp75j7Bh/kR0JKI" crossorigin="anonymous"></script>
  <script>
    (function() {
      alert("hola mundo");
    })();
  </script>
</body>
</html>