You have some choices -
The monthly cost for the 2TB iCloud Drive+ (in $US) is $9.99. See here.
The monthly cost of Apple One Individual plan is $14.95 which includes the 200GB iCloud + plan, as shown on this page
The monthly cost of Apple One Family plan is $19.95 which includes the 200GB iCloud + plan, as shown on this page.
The monthly cost of Apple One Premier plan is $29.95 which includes the 2 TB iCloud + plan, as shown on this page.
What happens to your iCloud storage when you sign up for Apple One explains what happens if you already have the 2TB iCloud+ and and buy the Individual or the Family Apple One Plan, (which as the linked page says: If the iCloud storage included in Apple One is less than your current plan): You keep both your current iCloud+ plan and the iCloud storage in Apple One.
...and separate billing for the two
The difference between subscribing separately to the Apple One family plan and an additional iCloud+ 2TB plan compared with the Apple One Premier plan is one cent.
However, you do save a bit more on subscribing separately to the Apple One Individual plan and an additional iCloud+ 2TB plan compared with the Apple One Premier plan - $5.01 per month.