If your bootstrap.js is not loading, you need to update your AppAsset file.
there is another asset bundle exists and it’s called BootstrapPluginAsset, you need to use it and your application will include bootstrap.js.

Your AppAsset depend section should be like:

    public $depends = [
        'yii\web\YiiAsset',
        'yii\bootstrap\BootstrapAsset',
        'yii\bootstrap\BootstrapPluginAsset',
    ];