Well, it is, but usually different companies make different types of cars. For example, Bentley makes luxury cars, which means they aren’t going to make minivans and small city cars. And Skoda makes budget family cars, so they wouldn’t be taken very seriously if they tried making a hypercar.
That said, both Bentley and Skoda are owned by the same Volkswagen Auto Group. If you don’t want to commit to only making a certain niche of cars, you can just make several car companies in the same thread and say they’re all owned by the same massive corporation. It’s kind of how my company works:
There’s a main brand for “nice” American cars - kind of like a mix of Pontiac and Oldsmobile - which I call Arlington; a cheaper brand called Callahan, which is like Chevrolet, a couple of more expensive and sporty brands, and finally a foreign (German) brand called Waldersee that sells cars that kind of feel like a mix of Opel and BMW.
Actually, that paragraph kind of explains how you’d think of making a company: Just combine some traits of a couple of real-life companies, come up with your own name and backstory, think if there are some cool engineering choices you’d really like to see in a car and voila. You have yourself a car company of your own. Make a first car for it, receive feedback from us other community members, and keep growing.