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>