The internal storage of your iPad is fixed - and cannot be extended or upgraded. The only way to increase your internal storage is to replace your iPad with another device with more internal storage.
By contrast, your iCloud Storage can be extended. Apple offer various subscriptions to extend your free 5GB, in a number of tiers (and monthly costs) up to 2TB.
Changing your iCloud storage plan is easy:
Settings > [Your Name / AppleID] > iCloud > Manage Storage > Change Storage Plan
Whilst your locally stored music cannot be moved from your iPad to iCloud storage, services such as Apple Match will allow you to access your iTunes music from the cloud.
https://support.apple.com/en-gb/HT204146