It’s usually engineers that work for the brand. I actually knew a couple of mechanical engineers that did a ton of testing new vehicles. They got to drive around in some cool stuff before it was released. They also “tested” competitor cars. The company would sometimes import cars from other countries that weren’t even released in the states. They’d basically beat on them for a period of time and note what they liked/disliked and then they’d literally disassemble the entire car. It was essentially reverse engineering higher level vehicles to see what their tech was and see how the could implement similar things into their own line of future vehicles.