You can add Bootstrap this way, follow below steps
1) Place your JS and CSS at below location
/app/design/frontend/vendor-name/theme-name/web/css
/app/design/frontend/vendor-name/theme-name/web/js
2) Call files in your default_head_blocks.xml
app/design/frontend/vendor-name/theme-name/Magento_Theme/layout/default_head_blocks.xml
add these line of code
<css src="css/bootstrap.css" />
<script src="js/bootstrap.min.js"/>
3) Run below commands
php bin/magento setup:upgrade
php bin/magento setup:static-content:deploy
more you can check here related to css https://magento.stackexchange.com/a/187029/54588
Note: This solution doesn't work with Magento 2.2.3 and above