Hey, I have a biological question which might relate more to lore than mechanics here.

As we know, several species are considered to be “goblinoid,” including: goblins, hobgoblins, and bugbears. The thing is, what does that mean? What is a goblinoid? Could we understand it as a separate phenotype or morphotype from “humanoid”? As in, humanoids (humans, halflings, etc.) walk upright and have a brain/cranium above their eyes, whereas a goblinoid walks in a stooped posture, may have arms longer than their legs, with a cranium, behind their eyes?

Or is the distinction one of culture and society? How does everyone understand these two distinctions?

I think goblinoids is just a catch all term for the races that Maglubiyet has conquered. Basically Maglubiyet went around and killed a bunch of gods and forced their races (hobgoblins, goblins, bugbears, etc.) to worship him and somewhat work together. That's why goblinoids look so vastly different from each other, because they're not related in any way other than culturally (loosely) and religiously. They all have vastly diffrent cultures, which is why I say they're loosely culturally tied because other than the hierarchy of bugbear > hobgoblin > goblin ,which is rarely usurped, they don't really have many connections. Bugbears are lazy slobs who enjoy an Epicurean lifestyle; hobgoblins are proud strategists and warriors who somewhat resemble samurai; and goblins are cowards who suck up to those in power while beating on those weaker. That is assuming Faerun lore, though. 

Quote from Ihsan997 >>

 

Or is the distinction one of culture and society?

This. The goblinoids are distinct races that were all subjugated by the god Maglubiyet. From Volo's Guide:

In bygone times the goblinoids were distinct from one another, with separate faiths and different customs. Then Maglubiyet came and conquered all who stood before him, mortals and deities alike. Gods and heroes who wouldn’t bend to his will were broken and discarded. He put his foot on the neck of mighty Khurgorbaeyag, bound the will of intractable Hruggek, and forced sadistic Nomog-Geaya to fall in line. What the goblins, the bugbears, and the hobgoblins were before their gods bowed to Maglubiyet no longer matters. Now they are, first of all, followers of Maglubiyet.

It's this shared worship that makes them a cohesive group and drives them to cooperate despite being very different races physically and culturally.

They're all still humanoid; that type is sort of a catch-all for anything roughly bipedal that has a soul and doesn't better fit into another type.