DeepWiki is useful for learn how Rails applications work. Here’s one for the rails/rails project.
Related to the above, I like to look at major Ruby on Rails application to see how they structure things and work. Some good examples:
GitLab.org / GitLab FOSS · GitLab: A massive, well-documented Rails app with a Vue.js frontend. It’s one of the best examples of a large-scale SPA built with Rails, though the codebase is complex and may be overwhelming for beginners.
GitHub - antiwork/gumroad: Heavy on the Typescript frontend, great application and a smaller startup.
While not directly related to learning Rails, I like keep an eye on the GitHub trending Ruby repositories as inspiration for seeing what people are building for the ecosystem.