I second the
1e 83 World of Greyhawk boxed set as the source to look at primarily.
It gives a brief breakdown on the 40+ nation/kingdoms, highlights of some of the gods which come in multiple pantheons, some breakdown of geographic features, organizations like prominent knighthoods, and other tidbits such as fantasy trees and rune symbols. Also the great evocative map.
Greyhawk is a bit of a kitchen sink, there are multiples of many things, but also not everything. There are four fantasy viking lands for instance, multiple Elven lands, multiple humanoid dominated areas, multiple different fantasy Arab lands, multiple barbarian areas, different pirate kingdoms, and such so it is not just the one pirate place or whatever.
On the flip side while there are dwarves you won't find any named dwarven kingdoms the way you will the elven ones. There is no fantasy East Asian stuff or fantasy ancient Egypt. It is a lot of diverse familiar evocative touchstones, but there is a not unlimited palette of them and you will not find everything. The specific mix has a flavor.
There are a ton of kingdoms in a big diversity of political situations and varying from bastions of good to a lot of neutral, and some are outright evil. A lot of kingdom neighbor politics and heavy tensions that drive the setting that can be background or worked in.
It has stand out evil threats of the Dark Lord recently freed demigod Iuz and his kingdom of humanoids, The humanoid lands of the Pomarj, the decadent decaying Great Kingdom which has been shedding breakaway states for a while, the behind the scenes evil manipulator monks of the Scarlet Brotherhood and some others like the humanoid kingdom run by human devil worshipers of the Horned Society.
It has the city of Greyhawk as a fantasy Chicago city state trade hub on a giant lake. It is also a D&D version of Lankhmar which is a sword & sorcery fiction version of New York. The mayor is a former member of the thief's guild and the city council includes thief and assassin guild leaders who are more powerful than the Lord Mayor. Also the previous former Mayor was an insane chaotic archmage who imprisoned Iuz for a century along with other assorted powers like the demon lord Fraz Urb Luu in a successful bid to ascend to become a demigod of wild magic.
It has the Wild Coast where it is a very D&D American frontier vibe where local places are independent a bit lawless and chaotic and do not answer to lords.
In the past there were giant super power competing ethnic empires that ended in the twin magical apocalypses of the Invoked Devastation and the Rain of Colorless fire.
