Nomulus is the platform we use to host all 46 of our top-level domains, including most notably .app which launched last week. Nomulus runs on Google App Engine and uses Cloud Datastore for data persistence. It handled the rush of registrations during the launch of .app just fine, handling a peak of 30,000 registrations in the first 3 minutes (that would be all the registrars sending in their preorders).
In addition to providing registration services, which only registrars interact with, the registry platform also powers the domain availability check seen on https://get.app and the domain WHOIS query seen on https://www.registry.google . These are the only services provided by the registry platform that the average person uses directly; everything else goes through domain registrars. Every time you create or update a .app domain at your registrar, your registrar is sending us an EPP command to effect those changes. See RFCs 5730-5734 for more info on the exact mechanism.
If anyone has any questions about the code, I can help with them.
It's not trivial to launch a new gTLD. No evil involved, just work.