If this question is more suitable for the anarchy101 forum (Subraddle?), just let me know.
Say you have a set of anarchist communities and there is enough interest from individuals in each community to pursue some large project. I'm thinking most of medical research but it could just as easily be autonomous vehicles or air traffic control systems or building a cellular network and the devices that go with it.
I can understand how self-organizing, non-hierarchical groups can tackle just about any project you can imagine at a community level. Construction, infrastructure, medical care, and of course food production and so forth.
But if you have 15,000 people that all are able and willing to devote some time to tackle problem X, how do you organize the work without introducing hierarchy?