I have a 60kW (peak) electrical system in my car, it is used to move the car (a hybrid), not the speakers.
OK, yes hybrids and EV's do have that sort of electrical power kicking around but not, as you say, to drove the in-car hi-fi.
Lots of batteries. It's not uncommon for guys that run serious amps like this to have multiple (5+) 100Ah+ 12v batteries in parallel installed close to the amplifier.
Hmmm, I can see that would supply the current, at least short term - I guess you'd easily get 2kA out of 5x100Ah batteries.
Mind you if a 20kW
capable amp were not grotesque enough the sound system is starting to look like it will render any vehicle impractical for normal use. I mean, speakers to handle 20kW are not going to be either small or light and you've just added 150-200kg of batteries into the equation.
Cars that 'demo' their systems for extended periods often also have multiple 300A+ capable alternators as well.
You'd need to, to recharge that battery bank.
The standard battery/alternator in most cars is only good for about 1500W in short bursts and about 500W continuous.
Pretty much what I said, I think.