iPhone 8 Plus iCloud storage

Not sure what you mean.

Please explain.

iCloud storage has nothing to do with the storage available on your iPhone.

5gb is the amount of iCloud storage that you get for free. You would get no more than that unless you pay for it.

To check the amount of storage on your actual phone (which is different from available storage on the cloud), go to Settings>General>iPhone Storage. This will tell you how much storage has been used out of your 64 GB and what it has been used by.