Lots has changed with Rails 6 and how stylesheets and javascript are managed.

My favourite article about this is this one by Ruby Yagi, and as a bonus includes how to make $ available in your app, as well as select2 and DateRangePicker.

This article provides a great introduction to the new ecosystem and how to include bootstrap and fontawesome into rails 6.

This was a very helpful guide for the rails setup.

This was a very helpful guide for setting up the layouts.

It is probably simpler now to just use yarn to add bootstrap to rails. See this article