Who decides what on Open Source Projects?

[BetterWater]

Hello friends,

How does one govern open source projects? By this I mean a project like bitcoin where several entities are working on the code repository?

What I don't mean.
Companies who open their up their source code and allow people to fork it.

How something like bitcoin is governed. Who decides who gets commit access on pull requests? How do we decide to remove someone?

I know the word meritocracy is thrown around a lot but in practical terms how do we measure meritocracy?