The way it worked a few years ago, at least, each year new tent-pole features were pushed down from the top. These features came from marketing, design, and Steve Jobs himself. There was always a much larger list of wants than possible for a given release, so the ever powerful hands of EPMs (engineering project managers) would help delegate the tasks and schedule a years worth of work.

The company as a whole loves small teams, they find them effective, and most managers are only given 5-10 engineers before there's pressure to split up into smaller teams.

During each start of a new release, engineers would pick and chose which features they wanted to work on. Depending on the team, some influential engineers and managers could fight for ranking the features they thought most important.

Features weren't always user visible things, one year, one of the tent-pole features was porting everything over to 64-bit. Of course this feature affected the entire software organization, so that radar was broken up into large teams, and then each framework and application had a radar.

Usually the person responsible for the feature at the team level was the DRI and anyone on the team could complete any of the radars associated with it, but the DRI was the main contact. They would be the direct line to the higher level radar DRI and so on. If at any point in the chain you didn't have NDA visibility, the EPM would be your contact.