It's easy to understand how to contribute being a dev: you look at the issues list on github and write code to fix them. 

However, it's not so easy to understand how you start contributing as a project manager in an open source project where there's no hierarchy nor regular jobs. 

Why and how the devs would want to be managed by me? I'd love to understand that better.  

 Because you’re not managing them you’re doing work nobody else wants, you’re a janitor! https://medium.com/all-things-product-management/product-manager-you-are-664d83ee702e 

 Ok, let's say I can and want to be a Bitcoin core PM. What would it look like for an open source decentralized project as Bitcoin? That's what I think it's not clear for me nor for anyone else.