Upgrading iCloud storage for iPhone 15 Plus with 128GB

You can not purchase extra storage for your phone; you can only purchase iCloud Storage.

To view and manage your iCloud storage, go here

If, in the future, you find yourself using the full 128GB capacity of your new iPhone 15 Plus, then you'll have to look at options such as using iCloud Photos, removing data from your iPhone (such as apps), or upgrading to a device with a larger capacity.

For example, you could save space on your phone by keeping low-quality/resolution photos on your phone but keeping the high-quality originals in iCloud. This would help you to save space on your device.

Take a look at this helpful article for more info Set up and use iCloud Photos – Apple Support.

You cannot upgrade the iPhone's storage capacity. That is set when you purchase it. iCloud is cloud storage on the Apple servers, and you pay for the capacity you wish to use. To get a larger storage capacity on the iPhone, you have to purchase a larger capacity phone.

iCloud storage (on Apple's servers) is not a substitute for device storage.

Things like iOS, apps, and any data that you need to access when you do not have an Internet connection (Wi-Fi or cellular) must reside in device storage.

Things that are synchronized to iCloud for the most part take up space BOTH in device storage (on every device to which they are synchronized) AND in iCloud.

If you are using iCloud Photos, you can enable "Optimize iPhone Storage'. This gives the phone permission to store lower-quality, space-saving copies in local (device) storage when storage space runs low. (The theory is that since the full-size originals are in iCloud, your phone can re-download full-size versions, on demand, later.).

That may let you "stretch" device storage – but aside from "Optimize iPhone Storage", there really isn't much that you can do about running out of device storage other than to delete or offload stuff, or to get an iPhone which has more device storage built in.