How do you figure out what and how to load the css/scss assets with Webpacker

For Bootstrap you added to application.scss(I may have named my folder differently:

import "bootstrap"
import 'bootstrap/dist/js/bootstrap'

and

@import '../css/bootstrap_custom';
@import "~bootstrap/scss/bootstrap";  // the tilde seems to be optional?

I reverse engineering this for an OpenLayers' module but the directory structures don't seem to follow a pattern and I plan to add other OpenLayers modules and wondered if how one knows what to add where? Although adding to application.js seem to work but javascript/css/application.scss is more logical.

gems were easier, but none current is available for OpenLayers. Seems to me application.js file should be in javascripts not buried in packs. Maybe I need a primer on Webpack.

Thank you for the great web casts.