Here is some information I found on computer operating systems used in cars. This info was posted by someone (not me) on another forum and is about 3 years old, so obviously things could have changed by now:
"Operating system in cars is outsourced by different companies, MICROSOFT, LINUX, QNX covers almost 90% of the market, till the date I haven't seen any company developing its own OS, they develop it from scratch or modify the existing one with the help of other companies it is outsourced from s/w companies. Even Intel, Nokia, Samsung is also trying to penetrate the market.
Microsoft -Windows Embedded Automotive 7 is the OS for In Vehicle Infotainment (IVI) systems such as Ford Sync, Kia Uvo and Nissan Leaf. Most widely used car OS today.
Linux
As ever, Linux is snapping at Microsoft’s heels. Back in 2007 Toyota announced that they were building a new operating system from scratch, expecting it to surface around 2015. Little more was heard on the matter until July when the motoring giant became a gold member of the Linux Foundation, donating $100,000 annually to further Linux’s cause. The GENIVI Alliance too is heavily invested in open source.
Comprised of many big daddies such as GM, BMW, Hyundai and PSA, they announced in Summer 2010 that they would be adopting MeeGo, an OS sprouting from the merger of Intel’s Moblin and Nokia’s Maemo.
Intel’s priorities seem to have shifted since then, more interested in a collaboration with Samsung called Tizen. That said, IVIs using one of these systems should be on the market in the near future.
QNX- My favorite

A commercial OS originating in Canada, QNX is a Real Time Operating System that prioritises the speed and predictability of a task. Specifically designed for embedded systems, it is used in the Mercedes-Benz COMAND center and for many of Harman Automotive’s IVI products.
I have heard apple is also developing iOS for cars but till now there is nothing as such in the market.
Clearly, much in the OS world will depend on how the software in our vehicles evolves. With consumers wanting to link their PC, smartphone and car, Microsoft’s dominance faces a serious challenge. This OS arms race is not just for car systems but for the future of computing as a whole. Companies are continuously looking for new cost cutting techniques with improved technology. So you never know when android start powering our vehicles' OS too."
https://www.quora.com/What-operating-systems-are-used-in-cars
Another person at the same link above says that most automakers are using Linux these days.