I have upgraded iCloud storage but how do I add my iPad to it. I have full iPad storage but 100gb free on iCloud

Here is what Apple states ➡️ What's the difference between device storage and iCloud storage? - Apple Support

Internal storage cannot be upgraded. What internal storage the iPhone or iPad ships with is what it will forever have for internal storage.

You can use iCloud as a place to offload stuff onto, but an under-configured-for-your-needs iPhone or iPad is never not going to need some internal storage management.

You don't. Your device(s) will always have the same storage as when they were purchased.