I phone storage management

You can manage your storage --> How to check the storage on your iPhone, iPad, and iPod touch

Manage your Storage --> Manage your photo and video storage - Apple Support

Your photos are always available on both iPhone and iCloud. You delete it on iPhone it is gone from iCloud and vice-versa. To save space on your iPhone use Optimised iPhone storage. Remember, if you delete in one it is deleted everywhere.

Get started with iCloud Photos:

1. Tap Settings > [your name] > iCloud > Photos.
2. Turn on iCloud Photos.
3. Select Optimize iPhone Storage to save space on your device.
4. 

With Optimize Storage, smaller, space-saving photos and videos are kept on your device while all of your original, full-resolution versions are stored in iCloud. And as long as you have enough space in iCloud, you can store as many photos and videos as you want.

How to check the storage on your iPhone, iPad, and iPod touch

Update your device to iOS 13.6.1, then read this support article: Clear Other storage on your iPhone or iPad - Apple Support

Keeping in mind that the “other” storage category is mostly filled with cache and system files created by Safari, the Mail app, other apps and iOS itself, that space is automatically managed by iOS and you can’t interact with it.

If the issue still persists, some benefits to reduce that space can be also obtained creating a full backup of your device, then restoring it to factory settings and, in the end, setting it up as new, as per these instructions: Restore your iPhone, iPad, or iPod to factory settings – Apple Support

When done, restore data from your previously made backup, then restart your device and check your device storage again.

For any further help, contact Apple Support directly, using this link: Get Support

Regards

Giulio