You can't turn off blocks of memory in a single RAM chip AFAIK, it's all or nothing. And I think enabling and disabling different RAM chips wouldn't be very viable since you'll need to move memory from one chip to another when turning one off, there would be a lot of potential issues when that happens.

However enabling and disabling RAM chips and rebooting the laptop, like a low-power toggle, would be possible.

Hot swapping RAM has been possible for a very long while in software, though originally built for servers. Apple, with their tight control over the hardware, could likely have also modified the power management controller and whatever other hardware that needed modifying to pull something like that off.