Displaying another entity's form on a node
\Drupal::formBuilder()-&gt;getForm() already returns a render array, see the FormBuilder::getForm() documentation.
To render the form, you just need to pass the results of getForm() to your $build array, then render it in your template.
In hook_ENTITY_TYPE_view():
function MYMODULE_node_view(array &amp;$build, NodeInterface $node, EntityViewDisplayInterface $display, $view_mode) {
  // ...
  $build['my_flagging_form'] = $form_obtained;
}

Then in your twig template:
{{ content.my_flagging_form }}

Or, if you're already rendering content in its entirety, you may not need to update your twig template at all.
As @4k4 noted in the comments, you can also add the appropriate meta data in hook_entity_extra_field_info to make your flagging form addition configurable (reorderable, etc) on your node's Manage Display screen. See Creating pseudo-fields in Drupal 8 for an example.