my iphone keep saying i have no storage but i have nothing on my phone

1.

Try to free up iPhone device storage see this Apple articles below:

If you need more space for an update on your iPhone, iPad, or iPod touch

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

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

How to check the storage on your iPhone, iPad and iPod touch – Apple Support (UK)

Clear Other storage on your iPhone or iPad

Clear Other storage on your iPhone or iPad - Apple Support

2.

What's the difference between device storage and iCloud storage?

What's the difference between device storage and iCloud storage? - Apple Support

Many time is not the IPhone, but the icloud storage who are full.

You could disable somes transfert/auto backup in setting->account->icloud.

You could check on icloud.com with a computer and our apple id and view if the icloud storage associate to our account is full.

Via the web page you could delete photo, video, contact etc who are in backup on icloud.

The maximum free icloud size is 5Go is not a big space and go full quickly.

In the icloud setting, on the Iphone, you clould disable Iphone backup to save space.

Dont forget, than if you flush files on icloud.com, the Iphone will sync and flush the file too.

If you want save data before flush from iCloud.com, you could disable apps backup on icloud in the Iphone settings.

Normally the phone will ask to know if you want let a copy on the Iphone.

You could use a 3 party application like imazing to backup the phone before delete the files.