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!