My understanding is that there's a final "executive review", which is almost a rubber stamp, in that only one or a few percent are nixed at this level. In the early days, supposedly it was literally Sergei and Larry. These days, I imagine it's the head of the division in question, or perhaps one of their direct underlings. Or, perhaps, (plot twist), they pump the entire thing through an ML model. :-)
One thing I've learned in my (long) career is that wealthy companies don't really need to care about how much they burn during the hiring process. (For starters, your labor is free to them at this point.) That's just the way it is.
Google's hiring process is set up to prevent as many false positive hires as possible, at the expense of false negatives. It's pretexted under the assumption that a bad hire is more detrimental/harder to fix than missing a few good hires. Eric Schmidt talks about it in How Google Works.
It's an intense process for sure. I interviewed with Google and made it through the process for a Linux admin job they had at Ashburn, VA, which was near to where I was living at the time (Reston). I had worked at the UUNet data center for almost 5 years at this point, but I took a job with a government contractor instead, as the pay was over 10k higher and the job a better fit. Google interview processes are no joke. It takes forever and the interview questions and practicals you get can make you doubt yourself. I don't regret not taking that job. I knew the guy who eventually filled that role and he said the job was very intense and very busy.
One thing I've learned in my (long) career is that wealthy companies don't really need to care about how much they burn during the hiring process. (For starters, your labor is free to them at this point.) That's just the way it is.