See: https://alexhofstaetter.at/query-custom-field-by-url-and-redirect-to-matching-post-in-wordpress/

add_template_redirect.php

// Try to get the post with the queried custom field value and do the actual redirect (or throw an 404)
function cfqar_redirect()
{
    if( get_query_var( 'qr' ) )
    {
        $posts = get_posts([
            'post_type'			=> 'product',
            'meta_key'			=> 'qr-code',
            'meta_value'		=>  get_query_var( 'qr' )
        ]); 
		 
        if($posts) {
            wp_redirect( get_permalink( $posts[0]->ID ) ); 
            exit();  
        }
        else {
            global $wp_query;
            $wp_query->set_404();
            status_header(404);
        }
    }
}
add_action( 'template_redirect', 'cfqar_redirect' );