I don't think "sheep" is a collective noun. The singular and the plural just happen to be homonyms, but they're still singular and plural.
I think a better example is "people". You don't usually say "peoples" unless you mean it in the sense of "nations" or "races".
But I think you're correct about "gli abbigliamenti".
They're pretty much synonymous but "clothes" tends to refer to a specific set of recogniseable clothes (e.g. "I have a lot of clothes in my wardrobe" or "I like shopping for clothes") whereas "clothing" is a more general term for any body covering (e.g. "cavemen didn't wear clothing" or "most clothing is made from fabric"). It's very subtle though, and as a native speaker it's hard for me to clearly explain the difference!
Because it's technically "Lo abbiglamento". A masculine noun that begins with a vowel always takes the article "lo". But it gets shortened. Lo animale --> L'animale Lo orologio --> L'orologio Lo uomo --> L'uomo And "lo" becomes "gli" for plural. Gli animali Gli orologi Gli uomini
How is it plural?
In English, "clothing" is an uncountable noun; you can't have "a clothing" or "two clothings", just "some clothing". Semantically, it's neither singular nor plural, but syntactically, it is singular: you say "the clothing is", not *"the clothing are". The same is true of Italian `abbigliamento'.
In English we can also say "clothes", which is also uncountable: you can't have "a clothe" or "two clothes", just "some clothes". This word is semantically neither singular nor plural, but unlike clothing, it's syntactically plural: "the clothing is" vs "the clothes are". Italian "i vestiti" works much the same way; the difference is that "vestiti" is also the regular plural of "il vestito", the dress.
I answered "i vestidi," accidentally misspelling "vestiTi," then proceeded to say, "You have a typo." The answer Duolingo provided was l'abbigliamento, as if it thought that was what I was trying to type. Not exactly a big deal, but still should be an issue to be looked in to.