If you want to seek a promotion, you can nominate yourself or others can nominate you. When that happens, a packet is assembled for you. It includes your self-review, and reviews from a number of your peers as well as your managers.
That packet goes to a committee likely made up of people who don't know you who then reach a decision about your promotion.
That's a simplification, but that's the basic idea. Your manager has a large influence on your success: to get promoted you need to show accomplishments, and that depends on you being given important work to do. But, compared to other companies, the process is much less biased by your manager. Your peers have a huge influence on your ability to get promoted.
