[Zarathust]

https://www.nasa.gov/feature/facts-and-figures

It is possible that code itself is not that complex, but the interaction between all modules certainly has a high level of complexity.

"In the International Space Station’s U.S. segment alone, more than 1.5 million lines of flight software code run on 44 computers communicating via 100 data networks transferring 400,000 signals (e.g. pressure or temperature measurements, valve positions, etc.)."