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:
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
External storage devices may be helpful for offloading data to which you do not require regular access - however, for the most part, this data cannot be directly accessed/viewed/edited/manipulated unless it is first copied back to the iPad. Despite optimistic marketing by Apple, iPad cannot replicate much of the flexibility of a traditional computer such as you may be attempting to make comparison.
Should you decide to investigate external USB Storage options you’ll require an Apple Lightning to USB3 Camera Adapter:
https://store.apple.com/uk/xc/product/MK0W2ZM/A
To provide sufficient power for the USB drive, you’ll need to connect a Power Adapter (charger) rated at least 12W to the auxiliary Lightning port of the Camera Adapter. The iPad, on its own, except for very low power devices, cannot supply adequate power for connected USB storage devices. The Power Adapter supplied with your iPad, with the appropriate cable, is likely to be adequate.
In all cases, your USB storage must have a single data partition - and be formatted as FAT32, exFAT or APFS. iPad cannot read multi-partition devices, or proprietary device formats (such as NTFS).
I hope this insight and information proves to be helpful to you - if only in promoting some additional though as to your needs. If you have further questions, please don’t hesitate to ask.