We're starting to bulk up on hypothetical questions about how anything centralized could happen when no one is in charge. The answers will always be imprecise and full of caveats and be primarily based on historical examples of how such scenarios have been handled in the past.
For that reason, I would suggest marking "how does a flat (non-hierarchical, anarchist, etc.) society build X centralized resource" as off-topic because the answer is either that the participants agree to do it together or they don't (and find another way to get the desired effect?)
Instead, I suggest we recommend rephrasing such questions as to what examples or proposals have addressed the need for X centralized resource—making them more historical than speculative questions.