I think you’re asking about how things are currently done, rather than under the draft proposals that have been shared, so I’m going to attempt to answer that question.
Ultimately, decisions get made by those who lead efforts. Richard makes the decisions about what gets included in Aeon. Shawn makes the decisions about what gets included in Kalpa. The team that’s working on Agama (the new installer) decides what’s important for that.
So we have a bunch of these “components” in the project that are run largely independently, but discussions organically take place when their interdependence causes an issue.
The board, ultimately, is a dispute resolution group. There are instances where they have done things beyond that scope (I couldn’t say what, this is just my understanding. I understand that is, in part, what led to discussions around the need for a bit more formalized governance structure).
That’s a very broad view, and as @sfalken said, when you start getting into more specifics, it varies quite a bit - and the differences rapidly become apparent more quickly. Broadly, “those who do decide” is the way it happens. But the definition of “those who do” also can be variable depending on what we’re talking about.