See also: https://blog.robsdomain.com/twitter-bootstrap-in-rails-6/
Rails 6 mades large changes to how asset compilation is done. In specific sprockets
has been removed for javascript processing and replaced with the common webpack
utility. Sprockets remains in usage for CSS processing.
When it comes to asset bundling, the "Rails way" is webpack for JavaScript and Sprockets for everything else. The default setup in a fresh Rail 6 install, similar to what Basecamp uses, still compiles CSS, images, and fonts with Sprockets.
This means, if you're a member of the Basecamp camp, all your webpack JavaScript source files would live in app/javascript and all your Sprockets CSS and images would remain in app/assets. Running rails assets:precompile will first build all the Sprockets assets into the public/assets directory, then will build all the webpack assets into the public/packs directory.