Source

File: wp-includes/theme.php

function get_header_video_url() {
	$id = absint( get_theme_mod( 'header_video' ) );

	if ( $id ) {
		// Get the file URL from the attachment ID.
		$url = wp_get_attachment_url( $id );
	} else {
		$url = get_theme_mod( 'external_header_video' );
	}

	/**
	 * Filters the header video URL.
	 *
	 * @since 4.7.3
	 *
	 * @param string $url Header video URL, if available.
	 */
	$url = apply_filters( 'get_header_video_url', $url );

	if ( ! $id && ! $url ) {
		return false;
	}

	return esc_url_raw( set_url_scheme( $url ) );
}