Ah, so when I see a sign saying "淡水/Tamsui", the first one is in Mandarin and the second one is romanized Hokkien. I assumed they were both from the same language.

Though it seems like hokkien-with-characters is a thing (from what you say, and also from what I've seen with 4-bopomofo ruby and Wiktionary), just less prevalent. Or perhaps one can look at hanzi at the same level as romanization for Hokkien (except romanization is more prevalent), i.e. "someone else's script that kinda works for us so we use it sometimes"

I'm surprised the extended bopomofo (I assume it's https://en.m.wikipedia.org/wiki/Taiwanese_Phonetic_Symbols which actually till now was the thing I thought was bopomofo) is used more as a writing system for Taiwanese.