Depends on who you ask. The following graphic is on the article about developing countries and includes emerging markets in the „lesser developed“ category: https://commons.wikimedia.org/wiki/File:IMF_advanced_economi...

Indeed. They may not be at the top when you sort countries by their metrics, but still great to live in.

As the the opposite to? And by what metric? Maternal mortality? Human development index? Access to clean water?

Depends where. For example Ukraine (before the war) had a similar gdp per capita to my home country, Morocco, which is definitely a developing country. Belarus is richer, but not by much. I guess those two countries are more of an exception now though.