Why my phone says iPhone storage full but I purchased 2tb in iCloud ,it’s like wasting money to add a storage in iCloud but still says full

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. 

See -> What's the difference between device storage and iCloud storage?

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

See also the following Apple articles:

How to use iCloud on iPhone -> How to use iCloud on iPhone – Apple Support (UK)

Manage your iCloud storage -> Manage your iCloud storage - Apple Support

Device storage and iCloud storage are different things.

iCloud storage is on Apple’s servers and is used for backups and for synchronizing data between devices.

Device storage is on your phone and is where things like IOS, apps, and offline media of any sort (whether music extracted from your own CDs, or offline listening copies of Apple Music subscription songs) live.

You cannot increase device storage by purchasing iCloud storage, and in many cases, synchronized data will require BOTH device storage and iCloud storage.

No, That's not how it works. Videos and pictures are saved to your device, not directly to iCloud. You need room on your phone to save them.

When you are using iCloud Photos, your phone saves photos to both places. Locally, so it can use them, and to ICloud, so other devices can pull down copies to their local storage.