Use Webpacker for Bootstrap/JS, and keep Sprokets for CSS and images (it is simpler).

Don't fuck about with where CSS etc goes.

Install:

  yarn add boostrap jquery @popperjs/core

Add to ''javascript/packs/application.js'':

  // Bootstrap 5
  import 'bootstrap'
  import "bootstrap/dist/css/bootstrap"

And test in a view:

  .alert-success Bootstrap!