All of it is, otherwise it wouldn't meet the 4 freedoms that define open source.
The 'project' maintaining the software may be centralized, but all its users "own" the software in the sense that the don't need to ask permission to the maintainer, and they can create their own modifications.
You're mixing a few different things. Free software and open source are different. and for each of them there are hundreds of different licenses that allows you to do something but not another.
Free software and open source are different marketing strategies for the same concept. The most commonly understood meaning for both terms is the same, from the very moment the Open Source Initiative was created.
The 'project' maintaining the software may be centralized, but all its users "own" the software in the sense that the don't need to ask permission to the maintainer, and they can create their own modifications.