How to get free storage in my iPad?

An Apple ID has 5 GB storage free. For more storage, subscribe to iCloud+.

https://support.apple.com/en-us/HT201238

iCloud includes 5GB for free. Anything beyond that costs a subscription fee. That is pretty much the same model for every online storage service in existence. A base amount may be free, but anything beyond that costs a fee, be it Google Drive, Dropbox, Box, or whichever service.