For adding Bootstrap in angular there is already bootstap included in the angular 9 in node_modules folder. otherewise dowload using npm via folloing command npm install --save bootstrap
install JQuery vianpm install --save jquery
After the installation of the bootstrap and jquery we must have to include in our project.So for this go to the angular.json file and then paste in styles
“styles”:[ “./node_modules/bootstrap/dist/css/bootstrap.css” ]
“scripts”: [ “./node_modules/jquery/dist/jquery.js”, “./node_modules/bootstrap/dist/js/bootstrap.js”]