How to theme a form with bootstrap I want to add much HTML to my form elements in order to arrange them on the page, I will be using bootstrap framework to do this.

Which method would be better, to create a twig template file or to use HOOK_form_alter?

If the way to do this would be to create a twig template file, then how can I ensure the form html tags with the form method is also rendered to the page? I have attempted to do this but I don't know how to get the actual form to render, just the inputs are there in the html source code.

I know in Drupal 7 that you had to render something called child elements, but when I check for this array in Kint there is no such thing.
A good starting point is the bootstrap theme. They have a stable version for drupal 8. (It's the most object oriented theme I've seen so far)


  So I would suggest to make your custom theme, as sub-theme of it.


If you want to do it fully by yourself, you would use hook_form alter existing forms (if needed) and theme_hooks/html.twig to add some bootstrap specific classes and change HTML structure.
The mentioned theme is a good starting point to look at all the things that need to be changed.