12factor.net then read the DevOps Handbook and Google SRE Book.
IMO P2P requires a completely different mindset compared to SaaS backend development. P2P is really more similar to desktop development. You won't be operating your app so you have to make it much simpler and more robust to run (NoOps). You won't have any PaaS or cloud provider services available. You have to tolerate version skew. You should optimize each copy for low scale (e.g. imagine a dozen apps running on a RPi).