al literally means "to the"
But it can be used in expressions where English would use: "of", "of the", "with the", and so on. It's one of those things that you just have to learn whether the correct prepositions to use is 'a', 'di', "in", etc... because they don't always map to their English equivalent.
This sentence (Sono bicotti al cioccolato) is probably closest to the English meaning of They are cookies with chocolate (ie cookies with chocolate bits in them). With is usually 'di' in Italian. But you use "al and not "del" (del = di + il) in this situation just because...
(Actually I don't know if there's a reason why. When I studied Italian at uni we were told you just have to learn them - sometimes they match up to the English choice, sometimes they don't)
In British English, maybe. In the USA, biscuits are something very, very different. Think of savory scones.
please show me the difference between chocolate cookies (made of chocolate batter/ dough) and chocolate chip cookies (plain batter/dough with chocolate bits mixed in) Also what chocolate cookies with chocolate bits mixed in would be. (Chocolate batter/dough with chocolate chips in it)
Because Duolingo is an American company and focuses on American English, so the course contributors have not added "biscuits" to the database. Feel free to flag the lesson and report "My answer should have been accepted".
Bear in mind, though, that in American English, a biscuit is something very different.
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How does "They are chocolate cookies" not make sense?
"I made some things for dessert."
"What are they?"
"They are chocolate cookies."
The existential "there are/is" (ci sono/c'è) means something very different and is not an appropriate translation.
"Hey guys, there are chocolate cookies in the break room."