The electronic parking brake is a motor that turns a screw that directly compresses the piston into the brake pads. Most cars do this to the existing rear brake pads and it doesn’t require any hydraulic pressure to work.

Some have a separate small caliper with the motor and no hydraulic connection at all.

The motors self adjust by measuring the load from the motor as it clamps down. Then rewinding a set amount when the brake is released. This can make it a pain to replace rear pads, since the screw will be extended too much to fit the new thicker pads/rotor. (You can rewind the screw with a 12v drill battery though and save a lot of time).

If all your rear pads were missing then you’d be screwed though!

And from what I heard the parking brake doesn't just lock up the wheels... It's not even connected to the front wheels. So you're missing most of the braking power. And that's also a reason why it's good for drifting.

Wonder how cars with electronic stuff deal with the legal requirement for there to be a second brake available.

The electric one will stop you (but engages all brakes, depending on car model). Have to pull it twice and hold it. Safety feature to prevent accidental engagement.