Check out example codes for "bootstrap 4 scss angular 8". It will help you in understanding the concepts better.

Code Example 1

# angular.json

{
  ......
  "projects": {
    "my-app": {
      ......
      "architect": {
        "build": {
          "options": {
            "styles": [
              "src/styles.scss",
              "node_modules/bootstrap/scss/bootstrap.scss"

Code Example 2

$ npm install bootstrap --save

Learn ReactJs, React Native from akashmittal.com