I suspect Boeing maybe treats software the same way as they've always done, just one of many boxes that need to be ticked in a long waterfall like process. They just throw more people at it hoping it gets done on whatever time line they imagine it needs to be done. They are likely to be very conservative with respect to tools, technologies, and practices (i.e. not much has changed there in decades).
The fact that SpaceX is able to come up with a new design for Starship and can then get it and the software working for several successful hovers & hops in around 9 months, tells me they are able to iterate quickly and adapt to quite major design changes. They launch a lot of rockets and they seem to be learning a lot every time they do this. I don't see how they'd be able to do this without very solid software development practices. I'd expect lots of CI & test harnasses, probably lots of emulators and other tools, and I have a hunch they probably use a lot of static code analysis etc. to prevent preventable bugs from happening.