install bootstrap angular 9
From angular cli:
npm install --save bootstrap
npm install --save jquery
Then add the following paths to angular.json:
"node_modules/bootstrap/dist/css/bootstrap.css" in the projects -> architect -> build -> styles array
"node_modules/jquery/dist/jquery.js" in the projects -> architect -> build -> scripts array
"node_modules/bootstrap/dist/js/bootstrap.js" in the projects -> architect -> build -> scripts array