We sync the shipment information of your customers' orders to Shopify, which means your customers can see it on their order status page.
You can also send an email to your customers when your order is shipped, Shopify has prepared some templates for this.
If you use these templates, your emails will automatically contain a link to your customer's order status page, where your customers can also see the shipment number.
If you have changed or replaced the Shopify templates, you can either reset them or add the link to your customer's order status page to your changes again, you can find more information on this directly at Shopify.