Hi,
"The item is on the shopping list."
"next (pronoun) - the person or thing in a list, series etc that comes after the person or thing you are dealing with now:"
1. What are the differences between 'on a list' and 'in a list'?
There's not really any big difference. These are often interchanged.
Here are a couple of comments on occasional small subtleties.
on a list - suggests a bit that you are thinking of the physical list, eg thinking of a piece of paper.
in a list - suggests a bit that you are not thinking of the list as a physical thing, but just as a sequence of names, eg 'your name is in a list in my mind'.
But, as I said, usually there is no difference.
Best wishes, Clive