@Stoori @ljwrites

* Binti series by Nnedi Okorafor. The first book opens with protagonist being one of the few humans accepted into a coolest university in the Universe; most of the people there are not humanoid.

* A Fire Upon the Deep by Vernor Vinge. The book opens with protagonist being the first human ever to be invited to work at a research facility / major node of galactic FidoNet; most of the people there are not humanoid; while most of the plot happens not in this research facility, the humans are not even the primary species to the plot, and not the only secondary.

* Xenogenesis series by Octavia Butler, humans are just yet another species that destroyed itself, and while there is no default species, Oankali (who encountered a lot of similar species) are much closer to being one than humans.

* Wayfarers series by Becky Chambers. While humans make a bit more than a half of the original crew, IIRC they are explicitly mentioned to not be a default species, and quite a lot time is spent dealing with other cultures (in a good way)

And as a bonus:
* Orthogonal series by Greg Egan. While it does not fit "if humans existed in the said world", I just could not help but mention it. The plot is set in a world with alternative physics, and as a consequence, alternative biology, from which an alternative and quite interesting social interaction emerges (and we witness a huge social change over the three books; I cannot say more without spoiling it).
It's just that I've seen too many other books which go like "what if humans but cats? They behave just like humans, they're indistinguishable from humans, but they walk on four feet, are covered with fur, and have tail and ears!".
And Orthogonal gives you an absolutely different yet so well-fleshed species.