Been studying standard HK Cantonese for almost 2 years at this point, can understand a wide variety of written Cantonese, news, stories, but cannot even read a basic mandarin sentence I see on wechat. Cantonese having 6 tones (with 9 in some regional variants). There are over 2200 different syllables in Cantonese, more than twice the number in Mandarin:
https://www.britannica.com/topic/Chinese-languages/Standard-....
I'd argue the majority of daily vocabulary is completely different, both in writing and in pronunciation, than its mandarin counterpart, such as some of the most commonly used words 而家，頭先，邊個，喺唔喺係，佢哋，講，有冇，點解，佢同佢講，畀 as in 我講畀佢聽，聽日，尋日，嗰陣時，嗰個，嗰啲， I could go on and on. Even after all this time, I didn't know the characters 是 or 哪里 until learning them a few weeks ago. Likewise, if I were a native mandarin speaker and heard the relatively simple, common sentence 佢哋而家喺邊度呀 spoken to me, I would understand exactly 0% of it. Grammar is significantly different with word placement, nuance of ending particles, usage of adverbial comparisons using Noun + V + 得 + 過 + Noun, using 畀，未。。。添, etc. I cannot understand a shred of mandarin when spoken to me, despite being able to communicate conversationally and understand most of spoken Cantonese from others. https://languagelog.ldc.upenn.edu/nll/?p=1211. There are varieties of sources analyzing the etymology of words in sinitic languages as well as their grammar structures and phonologies. Both languages are far from being mutually intelligible for monolingual speakers. Perhaps being a native speaker in both has allowed you to have a comprehensive mental-map of their isomorphisms.