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There was a NY Times article about it. IIRC there are ~12000 5 letter words and he got his partner to go through the ones she knew which was ~2500. Those 2500 were randomised to be the answers but the guesses can be any legit word. So the answer will always be reasonably common and there are loads of obscure words that never will be the final answer but are obviously still useful as guesses. But there is no reason skill should be more likely than swill or argon less likely than train. I think.
I thought I read somewhere that the list of Wordle words is made up of ones he thought his wife would know. I.e it is not a list of every 5 letter word (this might be wrong btw)