An iPad with 16GB internal storage is very restrictive. After subtracting the space required to install and run iOS/iPadOS, at most you’ll only have 5-7GB of accessible/usable internal storage.
It is impossible to increase the amount of RAM or internal storage in your iPad - as this is fixed during manufacture; what you have is fixed for the life of your device.
All AppleID accounts have 5GB of iCloud storage, this also being quite restrictive. While you can subscribe to a higher tier of iCloud storage, this is not a substitute for internal storage; iCloud storage can be used for files and data, but you cannot install Apps within iCloud storage.
Upgrade to iCloud+ - Apple Support
If your internal storage is full, your only option is to better manage the storage that you have - and delete unused Apps or unneeded data.
These support pages should prove to be helpful:
About storage on your device and in iCloud – Apple Support
What's the difference between device storage and iCloud storage? – Apple Support
Manage your iCloud storage – Apple Support
How to check the storage on your iPhone, iPad and iPod touch – Apple Support
Clear Other storage on your iPhone or iPad - Apple Support
“Offloading” Apps can be a helpful method of creating temporary space, if needed, for a software update.
https://9to5mac.com/2020/01/21/how-to-offload-apps-in-ios-to-save-space-without-deleting-their-data/
Additional information about creating sufficient space for installation of a software update can be found here:
If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support
External storage options can often help with managing limited internal storage. Depending upon your iPad model, the iPad will either have a Lightning or USB-C port - and may therefore require either an Adapter or USB hub to connect the USB Storage device. If your iPad has a Lightning port, you’ll need an Apple Lightning to USB3 Camera Adapter:
- Lightning to USB 3 Camera Adapter
https://store.apple.com/uk/xc/product/MK0W2ZM/A
