Wait, is earth supposed to be post apocalyptic? I don't really remember you actually ever making any lore posts about how earth is. Just that its basically being invaded by all kinds of aliens, friendly and sometimes not as it seems.
( the lore for earth is all W.I.P, so if this'll be the intended canon by the end of the year. who knows.)
There was a big event that lead to humanity being on the decline. Aliens saw earth as the perfect spot for interstellar travel. So, many species tried to establish contact and set up their own outposts eventually leading to earth becoming a new home for traveling aliens.
A majority of Humanity accepted the help and together; different cultures, species, people, all came and eventually absorbed the remanence into their cultures.
Any alien on earth now is referred to as a "NEO-earthling" while humans are called "Retro-Earthlings". Shorten that to "neoling" and "retling". So Earth is pretty lacking in sapient life, aliens fill the gap, and some carry on our aesthetics and culture. While others just so happen to align with what we do.