These were given as two example sentences on a site and I'm wondering if someone can explain the difference/which is right.
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Welcome to the site, Dan!
I don't know if "idiom" is the correct word, but the ...からできている is a phrase with a special meaning, in English "made (out) of ..."
I wonder if the above were example sentences, or a part of a question like "please select which one of the below alternatives is correct". Assuming the latter, the 1st one is right. (And means essentially means that dairy products are on sale on Thursdays)
But the sentence is clumsy and normally would not be used, as you realize contrasting "dairy products" with "products made out of milk". The ending よ indicates that this is probably told by a fellow consumer (and not eg printed on an advertisement). The the ending よ might best be translated as a leading "You know," or "FYI:"