How do I open up a YAML Form in a bootstrap modal window
The drupal way to do this would be to use blocks. The YAML Form module has the feature to put form in blocks. Place the form block in a region, where you want to have it on the page. Then style it in a block template:

block--my-form.html.twig

&lt;div class="modal fade" role="dialog"&gt;
    &lt;div class="modal-dialog"&gt;
        &lt;div class="modal-content"&gt;
            &lt;div class="modal-header"&gt;
                &lt;button type="button" class="close" data-dismiss="modal"&gt;&amp;times;&lt;/button&gt;
                &lt;h4 class="modal-title"&gt;Modal Header&lt;/h4&gt;
            &lt;/div&gt;
            &lt;div class="modal-body"&gt;
                {{ content }}
            &lt;/div&gt;
            &lt;div class="modal-footer"&gt;
                &lt;button type="button" class="btn btn-default" data-dismiss="modal"&gt;Close&lt;/button&gt;
            &lt;/div&gt;
        &lt;/div&gt;
    &lt;/div&gt;
&lt;/div&gt;