Sidekiq is used extensively for a multitude of background jobs, everything from audio/video post-processing to sending push notifications.
We use SNS for platform-agnostic mobile push message handling, essentially to avoid dealing with APNS ourselves.
We use GraphQL as our transport standard for all API calls.
We use PostgreSQL as our source of truth and main data storage.
We use Heroku to host our API endpoints, as well as our management interface.