Goblin & Gnome Lore
My personal D&D goblin lore is that most of their whole deal can be explained by the fact that they have a LOT of children, mature at about twice the rate as humans, and have about half the lifespan (if they live that long, which is rare), so they’re essentially to humans what humans are to elves.
Elves look at humans as unpredictable and hot tempered and violent and childish, and humans think the same things about goblins.
The rapid maturation makes them like overgrown, violent children, and their relative naivete makes them extremely prone to poor decision making and cult-like-behavior. The rapid rate of mutation also means that any given tribe of goblins is extremely different from any other tribe of goblins, with culture and slang almost incomprehensibly different between any given set of tribes in only a handful of generations.
The short lifespans and massive goblin redundancy give them a tendency towards chaos, lack of respect for life, and hedonism. There is no word for “safety” in the goblin language.
In more placid goblin settlements, adventurers will quickly find that most goblins do not have a well-developed sense of personal property - again, whoever owned that sword first probably died - which can create awkward situations even in peaceful encounters.
Goblin inventors, mages, and clerics tend to be extremely respected and feared within their communities: for the same reason that everybody listened to that kid who figured out how to get all of the extra lives in Contra. They rarely have the time necessary to truly master their art, but their recklessness can make for some wild results anyways.
Often they’re evil - in the sense a pack of schoolyard bullies who’ve been confused into worshipping an evil god would be - but they’re weaker than even low-level PCs, so they’re only aggressive in large numbers or with very obviously weaker foes, and as soon as the battle turns against them they’ll immediately drop everything to get out of dodge.
Relationships between humans and goblins are relatively rare for exactly the same reason that relationships between humans and elves are rare: thanks to the huge maturation gap, basically all relationships have Very Bad Age Dynamics that other people find creepy, like if you had a 45-year-old friend dating an 18-year-old. That would not be cool.
In contrast, we have Gnomes, who are small, overclocked elves.
Gnomes have the long lifespan, but their society can be goblinesque in some ways because gnomes are simply Very Fast Thinkers.
The experience reality at about 1.5x the speed of other cultures, like, everything around them seems frustratingly slow, so they give off really aggressive ADHD vibes.
That speed tends towards high DEX and INT scores because they’re literally wired faster than everybody else, but with the hyperactivity comes all manner of little issues like losing track of things around them, memory issues, and treating boring or repetitive tasks like the literal end of the world.
Gnomes are going to be the ones in your D&D universe to have developed calculus and established a legitimate space exploration agency in the woods, but also they often depend on humans for things like “walls” and “crops” because those things are, dare I say, BORING AS HELL.
A gnome game will involve several consecutive DC 20 INT checks and a chess-clock to keep things snappy. There might be rules that only take effect today, or while the wind is blowing in a certain direction.
A goblin game will involve live explosives.
An elf game is gonna essentially be cricket but worse: long and drawn out with loads of incredibly boring rules, some of them with justifications extending back centuries.