"The item is on the shopping list." [The item is (written) on the shopping list; The item is (included) in the shopping list; both okay, depends on the unspoken word]
"next (pronoun) - the person or thing in a list, series etc that comes after the person or thing you are dealing with now:" ["in" is used because it has a wider application (anything that is contained or included therein), but "on" means being included on the surface of something, such as writing]
1. What are the differences between 'on a list' and 'in a list'?