Too Long; Didn't Read
In Rails 6, it start to use "webpack" as default Javascript complier. By default, by default, Rails is still use the.application.scss/assets/stylesheets, which is not utilize the power of webpack. If you want all functionalities work in bootstrap 4, you might need to integrate jquery, and popper.js. You need to install gem and javascript package to make them works, I hope this can make this easier to do later.