I would guess the driver model works closely together with the hardware which in practice roughly means you get samples from the ADC and put them into a buffer

I would guess the driver model works closely together with the hardware which in practice roughly means you get samples from the ADC and put them into a buffer which is then directly accessible by the toplevel userland software. Which in turn puts output samples into a buffer which is then directly fed to the DAC. At least that is also how it works for ASIO or for example for data acquisition cards from major players like National Instruments.