Brake fluid needs to replaced eventually, same as in ICE cars. But that's only once in several years. No oil, no transmission fluid. And yeah, you need to refill windshield washer fluid every once in a while too :)
Other than that, since there are much fewer moving parts, there is much less regular maintenance necessary. Brake pads and discs survive much longer thanks to regenerative braking.
It has regen, which is basically energy capture via electric brakes. But it also has regular breaks, most drivers don't use them much so they last a really long time.
Yes and no. The low use of the brakes can be bad in non optimal weather, and can lead to the brakes getting stuck, uneven etc. There has been some problems with this in Denmark and Norway. So you still need to service the brakes.
Brake wear depends a lot on your driving style and circumstances. My very much ICE-only compact car develops a sideways brake bias because I never use the brakes very hard in normal driving, but you can alleviate that (and indeed, eventually must, to pass the annual inspection) by a couple of sharp hard brakings to clean whatever muck off the pads and rotors, or whatever that is causing it.
But I wouldn't call a once-in-a-decade service a hassle in maintaining a car. Without any luck you might have to service your suspension and track angles as often (due to potholes and whatnots) to keep your tyre wear in control.
How does it work? Does it use regen exclusively if you press the brake pedal lightly, and then the mechanical brakes are only engaged when you stomp on the pedal, or?
It depends on settings, state of charge and temperature of the batterypack. On normal it stars a slow regen as soon as you let go of the "gas". And brakes are only deployed if you hit the brakes. However if you already have a full battery (very rare) or the battery is cold (very often in cold winter) the battery cant absorb the charge, so regen is disabled. This means the car brakes very differently, with or without regen, but this is also indicates on the dash.
I have a Gen 3 Prius and it works the same. The only think I'll add is the Prius uses the brake pads at low speed (<10mph) and when you come to a full stop as well.
Other than that, since there are much fewer moving parts, there is much less regular maintenance necessary. Brake pads and discs survive much longer thanks to regenerative braking.