How do i get an iCloud drive?

There is a difference between iCloud+ plan and iCloud Drive

If you turn On iCloud on your iPhone you get 5 GB free iCloud storage.

What is iCloud -> iCloud+ - Apple

See also the following Apple articles:

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

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

Upgrade to iCloud+ -> Upgrade to iCloud+ - Apple Support

iCloud+ plans and pricing -> iCloud+ plans and pricing - Apple Support

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

How to back up your iPhone, iPad, and iPod touch

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

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See this Apple articles below:

Set up iCloud Drive -> Set up iCloud Drive - Apple Support                                                  

Set up iCloud Drive on all your devices -> Set up iCloud Drive on all your devices – Apple Support (UK)

Access and manage your files in iCloud Drive

Access and manage your files in iCloud Drive - Apple Support

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You can also 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