Ubuntu is a big project that has a lot of community involvement over many different aspects.
From who is in charge - to how decisions are made.
How is the work on Ubuntu organized and who organizes it?
It's explained at Ubuntu Wiki.
Summarizing what is there:
Ubuntu is baked by Canonical, so there is a team (that, in fact, are various teams) behind Ubuntu, and as many know, the leader is Mark Shuttleworth. This team decides almost everything about Ubuntu, although they do not do all the work themselves, here is where FOSS takes part in Ubuntu.
The community helps constructing Ubuntu, mainly through Launchpad, where people report bugs, release fixes and make the translations, the help is also made by actively participating in individual projects that Ubuntu makes use of, such as a
GitHub also plays an important role when we are talking about helping projects not just by translating and reporting bugs, but also by being a contributor developer.
To decide the steps that Ubuntu will take at each release they promote the Ubuntu Developer Summit that intents to bring together a diverse range of participants and to provide the perfect environment for mutual collaboration and best practice.
In order to keep communication for the sake of collaboration Ubuntu uses mailing lists in order to evolve with productivity and accordance.
To put all these pieces together and make Ubuntu happen is the work of the Ubuntu Teams.
So, this is how Ubuntu works!