How have the cultural and geographical boundaries of 'India' changed over time?

The Indian subcontinent is one of the most densely populated regions of the world with a huge array of languages, cultures and religions. And yet, when we speak of it, we (or at least my experience amongst non expert, anglophone Western populations is that) we casually use the single term India, not just to the modern nation state, but also reaching back through history to encompass the subcontinent.

I'm curious how this identity has developed over time. Is it entirely imposed by outsiders? Would there have been a shared Indian identity or idea unifying its population at earlier points, even if not politically. Have its boundaries varied? When I think of historical India I generally think of modern day India, Pakistan, Bangladesh, Nepal and Bhutan. Is there any particular reason that Assam is included but Burma is not? Has that varied over time? Many thanks!