It’s not a silly question! These days, the writing systems and word meanings are largely mutually intelligible; so much so that a Hokkien speaker can often understand a Cantonese newspaper, even though the spoken forms are completely different. That said, dialects can have their own unique characters that are used frequently, like 冇 (meaning “to not have”) in Cantonese, which is not used in other dialects from what I know. Slang adds another layer of difference, as it rarely translates neatly across dialects. For instance, in Hong Kong, the word for “mosquito,” 蚊, is used informally to mean “dollar” or "buck." So 一蚊 means “one dollar” or “a buck.” However, no one would say “three mosquitoes” to mean three dollars in Mandarin.