Would this be pluggable to any backend? Like say, Ruby on Rails or ASP.NET?

Triplit uses its own standalone server (similar to other database servers) which you can talk to via HTTP https://www.triplit.dev/docs/http-api

Is it built with pre-existing LLMs or did they created one from the ground up? With 21 million Seed A funding, an LLM powerful than GPT4 seems impossible. What am I missing?

Yeah seems like this could be replicated by another AI dev fairly easily.

Google launched another certificate.

How can one integrate it into his own android app?

To publish, just PUT/POST via your favorite HTTP library, or publish via intent (https://ntfy.sh/docs/subscribe/phone/#automation-apps).

And to subscribe, either use UnifiedPush (https://unifiedpush.org/), or use intents (see link above). Or, implement your own foreground service with WebSockets (just like the ntfy app does).