One of Tolkien's letters describes orcs as“squat, broad, flat-nosed, sallow-skinned, with wide mouths and slant eyes: in fact degraded and repulsive versions of the (to Europeans) least lovely Mongol-types.”
And I was thinking about, 1.) Look at the legs on these Mongolian wrestlers all decked out for a major national wrestling festival, and also 2.) how Tolkien's racist description wasn't enough, and orcs have been depicted as more and more grotesque over time. Canonically orcs more or less just look like humans, but that's not "other" enough so they keep getting turned in to more and more bizarre looking monsters.
Either way, this is what i'm thinking about when i'm laid up with the 'rona. If y'all like wrestling or buff men in tiny pants check out Mongolian wrestling. From what I understand it's a hugely popular sport there and has been for like a thousand years.
In The Tolkien Bestiary illustrated by John Blanche and Ian Miller, orcs are said to be corrupted elves, goblins are corrupted dwarves, and trolls are corrupted ents. Basically what happened to Smeagol and turning into Gollum. As elves/dwarves/ents are sealed away in darkness, tortured, and are given various concoctions and incantations, they are turned into orcs/goblins/trolls. They're analogous to fallen angels turning into demons.
I'm not sure how canon this is. It wasn't written by Tolkien himself, but it was an official product published only a few years after his death.
It's canonish. I think the elves were corrupted to orcs thing comes from a couple of different versions of Sillmarillion stories. Trolls being Ents is related to something Treebeard says, but Treebeard says they were made as a mockery of ents, not from ents. I think Tolkien talks about the idea that maybe stone trolls turn to stone in the sun bc they're not "really" alive, they're just cheap knock-offs because Melkor couldn't really create without Illuvatar's help. Whereas the dwarves are really alive because when Illuvatar found out Aule had made them Aule asked for Illuvatar's help so they could have fea and really be living and free willed.
Orcs and goblins is a linguistic thing. I think the Sindarin word for Orc is "yrch". The hobbit was originally written to be quite silly and approachable. I think the original edition mentioned something about riding a train to China before it was edited to be more clearly tied in to LotR. But The Hobbit uses much less Sindarin and more common english words like Goblin and Hobgoblin, but orcs and goblins are different ways of referring to the same sort of people.
It's canon for elves -> orcs, it was mentioned in SIlmarillion i think. No idea about ents -> trolls but it's plausible because Tolkien noted Morgoth lost power to create anything so for making trolls he had to make them from something already existing. About dwarfs -> goblins it's definitely non canon, goblins are just smaller and nimbler orcs who hate light even more, because they lived few thousands years in caves (remember that it is more than enough time in Middle-Earth for otherwise hundred thousand+ years evolution taking place).