The iOS-Frontend is UIKit and RxSwift. The backend is pretty lean — TypeScript, Firestore with the beginnings of PostgreSQL (with Prisma!). The recommender piece for the daily tips is Python. We also use cloud functions to run async updates (e.g. for daily updates and migrations).
Otherwise — lots of hosted services in the mix (Brave, Circle/Github, HoneyComb to name a few).
As for the future, we're exploring Flutter and a few other options to provide different experiences.
I am less interested in running k8s itself in Firecracker (or kata, or similar), but the pods started by it. I think there are advantages to that.