I found the solution and would like to share it with you all here:
// Erstellung der Beschreibung mit dem Titel der E-Mail, den Notizen und dem Kundenname
$description = !empty($service) ? $service['name'] : 'Unavailable';
$description .= "\n\n"; // Leerzeile
$description .= "Termin: " . date('d. m. Y | H:i', strtotime($appointment['start_datetime'])) . " Uhr\n\n";
$description .= $appointment['notes'] ?? '';
if (!empty($customer['first_name']) && !empty($customer['last_name'])) {
$description .= "\n\n"; // Leerzeile
$description .= "Interessent: " . $customer['first_name'] . " " . $customer['last_name'] . "\n";
$description .= "\n"; // Leerzeile
$description .= "Telefonnummer: " . $customer['phone_number'] . "\n";
$description .= "\n"; // Leerzeile
$description .= "Email Adresse: " . $customer['email'] . "\n";
$description .= "\n"; // Leerzeile
}
$event->setDescription($description);