As highlighted by developer “Khaos Tian” on Twitter, the
Studio Display only uses 2GB of its 64GB of internal NAND storage.
Some free space is likely needed for firmware updates, but the
62GB of unused space is seemingly otherwise useless at the current
time. Apple has not confirmed the quantity of the Studio Display’s
internal storage in its technical specifications.
This is one of those things that both makes perfect sense (the Studio Display contains an embedded A13 SoC and the A13’s lowest storage tier is 64 GB) and seems kind of crazy (the Studio Display has 64 GB of storage that is mostly unused and isn’t accessible to users). Effectively there’s a base model 9th-generation iPad in there.