120Gb worth of system data

I would ignore that because this system data will make room when you need more space on your iPhone.

On an iPhone, the term "Other System data" refers to a category of storage that includes various types of system-related files and data that don't fit into specific categories like apps, media, or documents. Some of It represents data that is necessary for the functioning of the device but doesn't fall into easily identifiable categories.

"Other System data" can include things such as system files, cache files, logs, system updates, diagnostic data, temporary files, and more. It's a catch-all category that encompasses a wide range of data used by the operating system and built-in apps.

Sometimes, the "Other System data" category can take up a significant amount of storage space on your iPhone, and it's not clear to me what specific files or data it comprises. If you notice that your iPhone's storage is almost full and the "Other System data" is occupying a large portion of it then...

• Restart the device. That's it.

If the above does not resolve the issue then...

You may --> Clear Other storage on your iPhone or iPad - Apple Support (IN)

Tap on System Data it will take you to "Other System Data"

Now the question is what is the "Other System Data" storage. Read on...

• Other System Data: Non-removable mobile assets, like Siri voices, fonts, dictionaries, non-removable logs and caches, Spotlight index, and system data, such as Keychain and CloudKit Database. Cached files can't be deleted by the system.
• System: Space taken by the operating system. This can vary based on your device and model.

About cached files in "Other System Data"

Finder and iTunes categorize cached music, videos, and photos as Other instead of actual songs, videos, or photos. Cached files are created when you stream or view content like music, videos, and photos. 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.

If storage on your device differs from what you see in Finder or iTunes

Since Finder and iTunes categorize cached files as Other, reported usage for Music or Videos might differ. To view usage on your device, go to Settings > General > [Device] Storage.

If you want to delete the cached files from your device

Your device automatically deletes cached files and temporary files when it needs more space. You don't need to delete them yourself.