Ask HN: Reference applications to idiomatically learn languages/frameworks?

I'm interested in seeing any languages, frameworks, and domains. Ideally these would be production applications and not tutorial apps. Here are some examples to get you thinking:

- A rails/spring/phoenix web application to learn how to structure code, handle authentication, front-end code, and common dependencies used

- A GIS application that uses an open source mapping library such as Leaflet, MapLibre, or OpenLayers and how that interacts with a frontend framework or state management library

- A Rust CLI application that demonstrates reading in parameters, error handling, and text processing