As far as i know cubase is allways working with samples in RAM since it is not diskreading while playing back a song or recording audio. (this is different from what other ROM Players (VSTI's with disk reading) or like harddisk-recorders do)
Best thing to refer to is the manual where on page 114 the limitations of recording audio are explained. There is clearly stated that recording is linked to the amound of available RAM. So my guess is that they stay there in RAM untill you save them somewhere.
I think the process of recording audio in cubase is in fact a form of 'hardware sampler' behaviour (RAM) and not harddisk recorder based.
Another thing to add is that with the audio parts in cubase you can do things in real time that you absolutely can't do with any harddisk recorder like realtime editing, realtime stretching, effects processing and so on. You would need a vast buffer setting that is not present in cubase IMO. So i would say audio handling is done entirely in RAM in cubase.
But let's see if someone jumps on to this with another view.
Also: the proces of audio editing and recording is non destructive, which means that when you load a song it reads the source samples one time and then stores them in a temporary memory location in RAM. (the pool)
Cubase 8.x, vienna ensemble pro 5,
Master:laptop Clevo, W170, i7 2760Qm, 16 gig, 2xSSD samsung 840pro/evo, +touch screen
Slave: desktop: Titanus, Q8440, 8 gig, 2x WD HDD + SSD
Sound: Motu Ultralite MKIII (usb)
control: Synergy, novation zero SL
vsti: Steinberg, East West, Vienna, Native Instruments, Spectrasonics and a lot of others