You cannot add physical storage to an iPhone after you buy it. Paying for iCloud+ increases your cloud storage, not the built-in storage used to install apps and keep local files.
Steps
- Open Settings and choose General > iPhone Storage to see how much device storage is left.
- If you need more iCloud space, open Settings, tap your name, then choose iCloud.
- Choose the option to upgrade or manage your iCloud plan and select the amount of iCloud+ storage you want.
- If the iPhone itself is full, use the recommendations under iPhone Storage, remove downloads, offload unused apps, or move photos and files elsewhere.
An iCloud+ upgrade can make room for backups, iCloud Photos, and iCloud Drive, but it does not change a 128 GB iPhone into a higher-capacity model.