Icloud storage

Where are you seeing this option for 100GB exactly?

Apple does not offer 100GB as an option for iCloud at all, so it sounds like what you are looking at is not actually iCloud.

Is it maybe Windows asking you to add storage for their own cloud service One Drive maybe?

One Drive and iCloud are separate services and would not share or use the same storage at all.

You should have the one you use. Both if you use both. I do not know which one you are really using.

Apple owns iCloud. Microsoft owns One Drive. They are separate companies entirely, and separate cloud storage options. Apple has nothing at all to do with One Drive. Different brands for a similar service.

You need to understand which storage you are actually using, and which by storage extension you would need to pay for.