Lots of good answers, but will clarify something for what it is worth:
All hybrid cars do have a 12V and a high voltage system. In my Prius (2009, 2003-2009 model), the 12v battery is in the boot.
The standard car electrics like computers (all 13 of them) and screens and headlight and indicators and everything runs off the 12v battery. The 12v battery is often a sealed lead-acid battery, to prevent risk of hydrogen gas explosions. SLA batteries have a long lifespan but tend to have lower maximum output currents. All they need to do is provide enough power to boot up the computer, which then activates relays that connect the (220-250v ish) high voltage battery.
The high voltage battery then powers a 2000w DC-DC convertor that charges up the 12v battery, and also provides power to all the electronics of the car.
The reason for this complex isolation of the high voltage battery is due to risk-aversion; in some ways it is like fast food coffee saying 'warning, contents hot'; or peanut butter saying 'allergy advice, contains peanuts'.... However in other ways it makes sense. The HV battery is usually near the rear of the hybrid, so power cables run to the motors at the front. Both batteries are DC, and 42v DC can kill a person, whereas many of us have had 240v AC mains shocks with no harm. In a car crash, or anything that disables the computers, the relays disconnect and the high voltage battery is isolated, so you don't die if you touch the car (hopefully!)
A typical alternator is 500-700 watts, so the 2000 watt 12v DC convertor is very good, and can provide very good quality power for audio installs among other things.
However, the battery in my 80s Nissan says 320CCA - cold cranking amps - which at 12 volts is around 3500 watts. Normal car starter motors can draw well over 2000 watts, putting strain on the 12v battery (much more expensive than normal car battery) and the DC DC convertor (extremely expensive, $1000s)...
If you try to jump start a normal car, it could destroy very expensive electronics, even if you do it right. Repair costs could be $1000s. If you connect it backwards it could write off a Prius, replacing 13 computers etc!...
If you do decide to help someone with a jump start, the best way is to connect your 12v battery in your hybrid to theirs, only when your car is fully switched on. Leave it for 5-20 minutes. Disconnect before they try to crank their engine and reconnect for more charging if needed.
With all that said, I jump started a friend's 2.5 diesel truck off my Prius. The 12v battery took damage, so if the fans are on, the voltage is too low to fully start the car's high voltage relays, and I get a 'red triangle' on the dash, so I have to switch off fans etc before I start my car! I was lucky only the 12v battery got damaged, and it hasn't got worse and it is 2 years later!
Be careful if helping others; whatever you do, don't let someone try to crank their engine while your car is still connected!