I think that to make Jelly then you need a substance that is known as 'pectin'. Most fruit does not contain enough pectin to set up. So a person can make a syrup from most fruits, but apples contain high amounts of pectin so when the 'apple syrup' cools then it becomes 'gelatinous' (https://www.dictionary.com/browse/gelatinous)
If the apples were not used then the person would need to go to the store and buy 'pectin'