Form - find rendering twig, debug enhancements

I'm trying to create a form. I am finding it difficult to determine which form.twig.html file is being used to render my form and where the css is coming from to render it.
I have a simple form with two text inputs, two buttons, and a date input. But depending on where in the file structure I put it, and what I name the file, I get different results.
I have not managed yet to get the date input to generate a date as described in the documentation.

Would it be possible to include some form of debug information, such as a debug log, in which the twig file that is used to render a page is listed?

Would it be possible to add something to the form tutorial about the css that is used to render a form?

To be honest when i'm debugging Twig templates, i usually just put in some text that will render in HTML to help ensure i'm viewing the correct one. Just a simple AAA or BBB in at the top of the content area of the template is usually enough.

Yes but which of the various form files, there seem to be three or four
sets in my setup (I am working off a Big Picture skeleton, but I have
considerably changed it).