I presume you're referring to powering huge sound systems though, with sustained heavy loads from the battery because of the alternators struggle to supply power on demand.

Not the same as a normal car or even a heavy use race car.

I've never used the cable you refer to, but welding cable is very flexible and has good think insulation. I find it ideal

Not sure what way you are comparing size for size etc ? 35mm^2 is conductor size only, not outer sheath etc and as I'm sure you are aware, proper multistranded cables can carry much higher loads than less strands with thicker strands

But using 50mm etc on a car is just insane unless it is for something as dedicated as a huge sound system

The only time a normal car sees very heavy current draw is during cranking. No other time.