As an alternative to "2nd/3rd" or "developing" countries, I really like the "Four Levels of Global Income" model[0] that Hans Rosling put forth in his book Factfulness[1] (a must-read IMO).
It gives a much clearer perspective on how people live (and as you mentioned, how similarly people live across the world amongst the same income levels)
It gives a much clearer perspective on how people live (and as you mentioned, how similarly people live across the world amongst the same income levels)
[0]: https://www.gapminder.org/topics/four-income-levels/ [1]: https://www.amazon.com/Factfulness-Reasons-World-Things-Bett...