How do I add storage to my xs?

It is not possible to add device storage to any iPhone. You may either purchase a new iPhone with additional storage or manage the storage you have.

How to check the storage on your iPhone, iPad, and iPod touch - Apple Support

You can't add storage to an iPhone. If you need more storage on your device, you will need a new device. You may be able to offload some items to iCloud storage to free up space.