In Firebug for Firefox, you can inspect an element as you describe, and then copy either the XPath or Css Path. This gives you the entire path you need right from html down to the specific element.

For example say you are inspecting a link with id myLink. You can copy the Css Path to get something like:

html body form table.myTable tr td a#myLink

Chrome has this functionality out of the box in the DevTools - see CSS Media Type Emulation.

There is a CSS file name near the selector (primary-unified-... for example)

Maybe there isn't. From my experince, if the class start with js- it means that it meant for handling js selectors. Something like: document.querySelector('js-paste-markdown').addEventsListener...

And you can see it in their code. For example, the class js-saved-reply-shortcut-comment-field:

See this tutorial: https://philipwalton.com/articles/decoupling-html-css-and-javascript/ (Search for js-*)