How do I buy more storage for my iPhone?

You can't buy more local (device) storage for your iPhone. Whatever amount of storage the phone had, when it rolled off the line at the factory is what it will have for life.

You can rent iCloud+ storage on Apple's servers. But that's for making backups of your iPhone, and for syncing data with other devices. In many cases, things stored in iCloud+ will also be stored on your phone (allowing the phone to work with its local copies when it is not connected to the Internet).

Apps and iOS upgrades are two of the things that need local (device) storage. You can't run them from iCloud.

If there's not enough space to download an App or update

If there isn't enough space to download and install an update, the software on your device will try to make space. It will remove only app data that it can download again and doesn't remove or relocate any of your data. If your device still doesn't have enough space, you can use your computer to update your device.

If you can't connect your device to your computer, you can make room for the update by removing content and apps on your device that you don't use. Go to Settings > General > [Device name] Storage.

Learn more about the space your apps and content use.

An iPhone has a limited Drive Capacity ( SSD ) that can not be changed to a Larger Capacity Drive SSD

Hey Yordi92!

Glad to help, if possible. This is the macOS section of the website. Anyhow could you please elaborate on what happens when you try to download an app, and what storage is preventing you from doing so? And in particular, any error messages would really help! Cheers!