We don't use AI on a wide scale like that, and I don't think AI is quite there yet. I use AI in a few ways, but you always have to check the output as it's often wrong, or uses method names, and database table columns that don't exist.
I use Github Copilot which has access to our Github repository so it can learn about our code and make sensible suggestions from either prompts, or auto-complete, for example:
CleanShot 2024-07-18 at 09.01.20.mp4
I sometimes use ChatGPT to avoid looking up syntax or functions and to avoid a bit of trial and error which probably saves 5-10 minutes on a bad day.
These tools can help a little, but they don't really wholesale fix issues and find errors. I think that'll come eventually.
During development of Invision Community 5, we have several safeguards in place to minimise bugs. Every single change is put into review via a pull request before it is merged into the development branch. The review has an automated PHP lint tool that checks for syntax issues and won't allow it to be merged if it contains them. We also do a human review on not only the code, but also the change to make sure it does what we wanted it to do. Some of these reviews go back and forth for weeks, others are merged quickly.
That said, bugs are part of software development, that's why we have alpha and beta testing. The v3 to v4 transition was complicated as it was a complete rewrite from scratch, we had a much smaller development team and it took much longer than originally planned.