[daniel_reetz]

It's a little deeper than this, software for each module is typically provided by a tier 1 or tier 2 supplier according to a spec provided by the OEM. Sometimes the tier 1 or tier 2 supplier is also subbing out the software or stuck with some system on chip that sucks.

So for a made-up example, GM wants to build a smart dash in the latest SUV, maybe Bosch or Continental has one with a SoC inside and their own software hell. OEM works with supplier to integrate, bugfix, skin, and customize. But they don't write it from scratch.

[caskstrength]

Yes, and suppliers outsource the actual development and testing to cut costs even further.

[ahartmetz]

AFAIK, car manufacturers want to bring more software in house as a core competency, which is probably good because the "Tier 1"s are generally even worse at software than them and have worse aligned incentives.