Yii doesn't wrap the bootstrap basics into PHP code since HTML is very simple by itself in this case. You can find details
about using the basics at bootstrap documentation website. Still Yii provides a
convenient way to include bootstrap assets in your pages with a single line added to
AppAsset.php located in your
public $depends = [ 'yii\web\YiiAsset', 'yii\bootstrap4\BootstrapAsset', // this line ];
Using bootstrap through Yii asset manager allows you to minimize its resources and combine with your own resources when needed.