OS System storage?

Then restoring your iPhone is the last option. Given the size of your 'System' storage (137GB), I'd suggest backing up your phone to ITunes and/or iCloud, restoring your phone to factory settings, then restoring your data from the backup.

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

About backups for iOS devices - Apple Support

How to back up your iPhone, iPad, and iPod touch - Apple Support

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Did you click on the links I provided, and read the articles. (The text in blue is clickable) If so, did you try restarting your phone, as suggested in the first article. Restart your iPhone, iPad, or iPod touch

Did you try the following steps, as discussed in both articles?

1. Open iTunes on your computer (make sure that you are running the latest version of iTunes)
2. Connect your device (your iPad or iPhone) to your computer (Mac or PC) via your USB cable
3. Follow the onscreen tips, like entering your passcode or tapping “Trust This Computer”
4. Select your iPad or iPhone in iTunes
5. Now just wait, you do not need to do anything else. Just wait several minutes
6. Now again check current system storage to see if it is reduced

If neither of those solutions help(ed), then restoring your phone is the last option.

As I said in my first post, follow the steps in the articles below on how to free up 'System' storage.

“System” Storage And What Can You Do To Clear - macReports

How to Reduce “System” Storage Size on iPhone or iPad - OSXDaily

Then Contact Apple Support or make a Genius Bar appointment.

Follow the steps in the articles below.

“System” Storage And What Can You Do To Clear - macReports

How to Reduce “System” Storage Size on iPhone or iPad - OSXDaily

iPhone, iPad storage Vs System Storage

Recognise components of iPhone storage contents

When iPhone shows 30.3 GB of 32 GB Used connote only 1.7 GB of storage is unused (refer Pic below)

When you scroll down to the bottom of the app wise storage list one comes across an isolated band showing

• System Storage 8.24 GB (See Pic below)

• Let us understand this that this iPhone has only 1.7 GB of storage left. Ergo the system storage is part of the used storage. In other words System Storage is subset of Storage used.
• Category Colour band has following colours depicting categories as listed below
  • Red Colour : Apps
  • Light Blue : iCloud Drive
  • Green: Messages 
  • Yellow: Photos
  • Dark Blue: eMail
  • Grey: Others
  • Orange: Books
  • Blue: Songs
  • Purple: Media / Video

Majority of the System Storage is taken up by System Software & Apps. In this case Apps have 5.62 GB and Others which has 2.74 GB (party used by system storage) can be seen from the iTunes category band when the same iPhone is connected to iTunes on Mac.

System Storage has many threads in most of the App data, eMail Accounts, Cellular Data, Messages, Photos, residuals of deleted apps or corrupt apps etc.

Watch the video which I have made and uploaded on my YouTube Channel

https://youtu.be/Q4erA_acy14

About storage on your device and in iCloud - Apple Support

https://support.apple.com/en-in/HT206504

Please provide screenshot of the first page of iPhone Storage and the Last Page. (Wait for 30-40 Sec on iPhone storage section before taking Screenshot.)