iPhone Storage Vs iCloud Storage
When you get a popup message on your iPhone or iPad that the storage is full.
Look closely storage of iPhone or iCloud
iPhone Storage (Setting > General > iPhone Storage)
- Total storage remains same through out life span of the iPhone
- Storage can not be enhanced
- You may take back up on Mac or PC using iTunes without iCloud
iCloud Storage (Settings > [Tap your name] > iCloud)
- By default every user gets 5GB of free storage space
- You can buy more storage based on your requirement 50, 200GB, 2TB with monthly subscription
- Based on your storage capacity you can back up iPhone or iPad
- The space can be shared between your family members for storage plan beyond 200GB
About storage on your device and in iCloud - Apple Support
https://support.apple.com/en-in/HT206504