To put it bluntly, they make things up. Same approach as virtually every other game really - the manufacturer publishes some claimed performance stats, and the handling artists fill in the blanks to make something that feels appropriate. This doesn't even just apply to concept cars either, there have been tons of cars in the Forza series that use placeholder data or things copied from other cars to fill in gaps in their research.
As for the sounds, most cars in games don't use authentic sounds anyway - outside of really specific-sounding cars like the WRX, Huracan, and Viper, most sound sets are mixes of various cars that were available. The XJ220 sound was historically a 300ZX recording for example, because it's still a turbo V6 and sounds right enough. With FH5's newer sound tech the number of recordings available reduced somewhat, which leads to a lot of similar-sounding cars (e.g. the very buzzy inline four that's on most hatches with exhaust upgrades).