If their idea is both novel and useful, and if it actually works, and they can actually produce it, then: They can sell it to other automakers.

(GM has made a lot of cars with their own transmissions. And at various times, they've supplied -lots- of them to other automakers all over the world. They've made a lot of money doing this.

Someone's gotta build the machine vision/control systems for all of these self-driving cars; that someone may well be Rivian.

It's not as sexy as something like a new convertible might be, or a $40k self-driving electric car, and a consumer might not even know that the new car in their driveway has expensive Rivian parts buried inside, but that future can be very profitable for them.)