How do I buy storage on an iPad mini 4

I thought you may have a 16 GBs iPad.

You should never, EVER fill up ANY computer or computing device completely full of data!

You still may NOT have sufficient storage for recording any videos.

Here's why.

The amount of available, practical, usable storage is based not only on how much iOS/iPadOS takes up but on the actual size that 1.0 GB is equivalent to, which for Apple 1.0 GB ≈1.075 GBs

Also, iOS/iPadOS devices need to maintain a minimum constant of between 2-4 GBs to ensure nominal, expected performance and function of iOS/iPadOS.

Also, my “current” installation of iPadOS 13.2.3 takes up approx. 5.5 GBs of my iPad's internal storage space.

Your version of iPadOS 13 maybe taking up more internal data storage than my version!

So, with all of this stated,

A 16 GBs storage iPad starts out, after initial drive formatting, at approx. 14.5 GBs of free, internal storage.

After the installation of iPadOS, at nearly 5.5 GBs, leaves 9 GBs of of free, internal data storage space.

iOS/iPadOS needs a minimum of 2-4 GBs of constant remaining storage, AT ALL TIMES to insure/ensure nominal, acceptable operation of all iOS/iPadOS functionalities.

This leaves a 16 GBs iPad with ONLY a practical, usable storage space of between 5 and 7 GBs AND THAT IS IT!

This is virtually NO internal data storage to do much of anything with that iPad!

This is why Apple quietly, silently did away with all 16 GBs iOS device back sometime in 2016.

Compare this to a 32 GBs storage iPad starts out with approx 30 GBs, after internal drive formatting.

Minus 5.5 GBs for iOS leaves 24.5 GBs.Minus 2-4 GBs for iOS iPadOS system reserve equals approx. between 20.5- 22.5 GBs of usable data storage space.

This is OVER 3 times the usable, practical internal data storage space of a 16 GBs storage model iPad.

You can’t add more storage to a device after purchase.

What is the rated internal data storage size of the iPad you are using?