(Disclaimer: I work at Amazon, but don't speak on behalf of the company. Just sharing some bits about my experience, this is just my experience and doesn't reflect official Policy.)
Prior to working at Amazon, I had mostly worked for start-ups. The thing I found at startup is that people are trying to figure out processes, and it's kind of hard. Most people putting in a process (whether that's HR or the founding team) are trying to do their best, but start-ups aren't at a large enough scale where they really need to have scalable processes, and for individual contributors all the distinctions are pretty informal anyway, so there's usually no individual contributor promotion path.
At Amazon, I was fortunate to work under someone who had been my boss previously, and we had a good working relationship. He was not afraid to give me blunt, useful feedback about either my work or how my work was perceived.
At some of the start-ups I worked with, part of the year-end evaluation involved filling out a form with a lot of evaluation criteria - the employee would fill out a self-eval form, and the manager would fill out a form, and then they would compare during the evaluation meeting. There's no such equivalent at Amazon.
One thing that overall I like about Amazon's performance management in terms of promotions is how it's centered around a document that you and your manager write together. There are some tedious aspects to the process, but if nothing else when you go up for promotion you know what your manager is officially telling other people are your strengths and weaknesses. The flip side of the process is that decisions about your promotion are being made based sometimes on the quality of your promo doc. I'm not sure about all the company, but at least in the team I was on some of the senior folks set aside time for reviewing promo docs that they were working on with other members of the team to iron out any "doc writing" issues.
We don't really have an official OKR system, but as a more senior engineer my personal goal has generally been to enable whatever my team's goal is for the quarter or year (launch this product; get this architecture document done; research how to solve this problem).
Besides the official process, I've also found it useful to ask trusted peers how I'm doing, whether I'm giving them everything they need and expect of me. It's also pretty common to have one or more mentors outside of your team to give you more informal feedback.