Eh i can't say for sure how it works but the drive key i believe is in the drive firmware, it checks the key on startup of the drive hopefully with some heavily encrypted communication with kernel parts of the system when booting, but it doesn't need to actually send the key itself across ever? The drive can 'verify' it matches without actually sending the bytes. (it certainly doesn't dl the fw off the drive every time to heck im sure??) I'm not fully certain i used to read a fair bit and follow the 360 progress early on and luckily in that case you could just pull it off the shitty pc drives used by MS to cut costs with certain commands that dumped the firmware. Things on 360 got more complex later on... in
this case the system can pull the drive key apparently (so there's some other hack at work surely usermode can't do that) and maybe even attempt to replace the firmware during updates like 360 eventually could do.
I don't know how Wii worked but apparently this is pretty similar except maybe they can update the firmware on the drive now/? Hopefully the Key has maybe 2 banks to store legit and 'hacked' firmware in case of future countermeasures.
To clarify the WiiU key only needs the drive key to work. But it needs a trick to obtain that key on the system. After that the Wii-U can just keep asking the drive 'are you legit' is this a legit disc'? and the drive spoofs every response.