The main difference was/is that at Google your immediate manager, director, PM, peers and everyone else in your product unit (who you work with every day) have almost zero say in whether you get promoted or not. You have to essentially summarize everything you did in bullet points and send it over to an anonymous committee who don't know who you are. They will base their decision on this piece of paper without any additional background or context.

This does help in various ways – the process is more objective, there is less bias, less departmental/managerial politics etc. The drawback is that a lot gets lost in translation. There is too much burden on you as an engineer to pick and choose what you spend your time on so it looks good to the committee.

In other companies I have worked at getting promoted was a byproduct of doing a good job. At Google getting promoted is the job.