How to increase storage on iPad not just icloud?

You can't in the iPad itself: the storage is built-in and cannot be modified. However there are other options - please see:

Connect external devices or servers with Files on iPad - Apple Support

You can't and you don't. If you are running low on storage, your only solution is to delete things you don't need or use such as Photos, Videos, Apps, Email, Texts, Files, etc. Or you buy a new iPad with more memory.

iCloud by the way, is a syncing service and no matter how much space you buy for iCloud backups, it will never increase the memory on your iPad.