You can use the wp_mail() function in your theme’s functions.php file or in a custom plugin.

function send_custom_email() {
  $to = 'recipient@email.com'; // Email address to send to
  $subject = 'Hello from WordPress'; // Subject of the mail
  $message = 'This is a test email sent from WordPress.'; // Email content
  $headers = array(
    'Content-Type: text/html; charset=UTF-8',
    'From: sender@email.com'
  ); // Sender email
  
  //Send the mail
  wp_mail($to, $subject, $message, $headers);
}

$to: Recipient’s email address.
$subject: Subject line of the email.
$message: Body of the email.
$headers: array of additional headers like Content-Type.