Immediate boss has some input, and then everything is bounced 2-3 levels up the line for final decisions. Sponsors rescue their favorites, managers talk about old baggage for people that is totally unrelated to recent contributions in order to provide some reason to throw them to the bottom, etc. Few in the room know you unless you have been around a while with some exposure to senior management.
Often one negative comment from anyone sinks you unless someone else speaks up to save you, which seldom happens. Disagreements are worked out on the sidelines. Usually involves I'll support X if you will support Y, and then I am ok with cutting Z. Fill the buckets and they get lunch.
Then HR and Legal run the disparate impact analyses and recommend necessary adjustments to get everything bullet proof in terms of diversity, discrimination, etc. A few tweaks are made and it's done.
Everyone has plausible deniability if they all keep quiet, which they do. You might be surprised at who sc--w*ed you. May be someone getting back at you for something that happened ages ago, not necessarily anyone in your direct line of reporting.
That's how EM bakes the cake.