Storage

Note there's different kinds of "storage". Click on this link to discover what's the difference between device and iCloud storage, ways to manage your space, and upgrading to iCloud+ if you need more --> What's the difference between device storage and iCloud storage? - Apple Support

Take-away:

1. Storage on a device itself is fixed and cannot be upgraded. If you want that then you have to change your present device for one with more capacity.

2. iCloud storage is on Apple's main computers. It is not a like a normal storage hard drive but by using optimization settings on your device it can help free up device storage. Lesser used items and larger file versions can be stored in ICloud and you can get access while connected to the Internet. It does not turn a 64 GB storage device into a 64 (or whatever) + 50GB/200GB/2TB device but if you have a lot of things that aren't being used much, or you don't mind only having lesser quality photos on your device, it can free up extra space so you can make the best of the GB you do have on the device itself.

If you have upgraded iCloud storage and are seeing a message on your device that storage is full then it is possible you are not making the best use of iCloud storage and need to use optimization features. If you do have optimization activated then it is possible you are exceeding your device's needs in terms of on-device storage and you need to buy a device with more storage.

Click on the link to learn how to optimize the storage on a device --> "How to check the storage on your iPhone, iPad, and iPod touch" - How to check the storage on your iPhone and iPad - Apple Support

- "If your device is low on storage, it automatically frees up space while installing an app, updating iOS or iPadOS, downloading music, recording videos, and more.

To make more storage available, your device can remove some of your items, like streamed music and videos, files in iCloud Drive, and parts of apps that aren't needed. It also removes temporary files and clears the cache on your device. But your device only removes items that can be downloaded again or that aren't needed anymore."

I am not familiar with "system data" but it may be what used to be called "other". Here's information I can find about dealing (or not) with "other" storage on i-devices:

What's in Others in your iCloud storage --> What's in Others in your iCloud storage - Apple Support

How to check the storage on your iPhone, iPad, and iPod touch - Apple Support - How to check the storage on your iPhone and iPad - Apple Support - "About cached files in Other ... When you stream music or video, that content is stored as cached files on your device so you can quickly access it again. Your device automatically removes cached files and temporary files when your device needs more space." "You don't need to delete them yourself." (Which I interpret to mean you cannot delete them directly yourself.)

Clear Other storage on your iPhone or iPad --> Clear Other storage on your iPhone or iPad - Apple Support

How to Clear "Other" in your iPhone's Storage - Use Backup

How to Clear "Other" in your iPhone's Sto… - Apple Community

- A somewhat drastic but oft mentioned solution achieved by making a backup of your device which will not include "other", erasing the device, then restoring the device from your slimmed backup.

Clear the history and cookies from Safari on your iPhone, iPad, or iPod touch

Clear the history and cookies from Safari on your iPhone, iPad, or iPod touch - Apple Support

What is "Other" and What Can I Do About It? - What is "Other" and What Can I Do About I… - Apple Community

Check the cellular data usage on your iPhone and iPad - Use cellular data on your iPhone or iPad - Apple Support

- "To see the cellular data usage for individual System Services, go to Settings > Cellular or Settings > Mobile Data. Then scroll to the bottom of the screen and tap System Services. Cellular data can't be turned on or off for individual System Services."