Install Bootstrap:

npm i --save bootstrap @popperjs/core
npm i --save-dev sass

Create bootstrap.scss with the below content:

// Required
@import "~bootstrap/scss/functions";
@import "~bootstrap/scss/variables";
@import "~bootstrap/scss/variables-dark";
@import "~bootstrap/scss/maps";
@import "~bootstrap/scss/mixins";
@import "~bootstrap/scss/root";

// Components
@import "~bootstrap/scss/_buttons.scss";
@import "~bootstrap/scss/_tables.scss";
@import "~bootstrap/scss/_alert.scss";
@import "~bootstrap/scss/forms";
// ...

The list of components could find at node_modules/bootstrap/scss

Then import bootstrap.scss to main.js:

import 'path/to/bootstrap.scss';