Fly, launched in 2017, is a platform for applications that need to run globally. It runs your code close to users and scales compute in cities where your app is busiest. Write your code, package it into a Docker image, deploy it to Fly’s platform and let that do all the work to keep your app snappy.
Postgres on Fly is a regular Fly.io app, with an automated creation process and some extensions to simplify management. It relies on building blocks available to all Fly apps, like flyctl, volumes, private networking, health checks, logs, metrics, and more. It is relatively trivial to enable multi-region PostgreSQL with read replicas.
At one point apparently managed Redis was offered as a separate product, but now it is still possible to run Redis on Fly as a standard app, but you have to manage it yourself.
The Fly.io blog is both informative and hilarious, excellent reading.