My iPhone storage is just 1 gb but showing 13 gb

You cannot add storage on any iOS device, however you may buy iCloud Storage for taking backup and keeping a copy of your important data such as Contacts, Calendars, Bookmarks, Mail, Notes, Voice Memos, shared photos, iCloud Photos*, Health data, etc..

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

Look closely storage of iPhone or iCloud

iPhone Storage (Setting > General > iPhone Storage)

1. Total storage remains same through out life span of the iPhone
2. Storage can not be enhanced
3. You may take back up on Mac or PC using iTunes without iCloud

iCloud Storage (Settings > [Tap your name] > iCloud)

1. By default every user gets 5GB of free storage space
2. You can buy more storage based on your requirement 50, 200GB, 2TB with monthly subscription
3. Based on your storage capacity you can back up iPhone or iPad
4. The space can be shared between your family members for storage plan beyond 200GB