I’ve studied both and let me tell you, I’ve come across people here who are completely amazed that they’re different. Back when I was in middle school I met a guy who was amazed that Japanese and Chinese people couldn’t understand each other’s languages, and in high school when I demonstrated the differences in sound to a few of my peers they were astounded to discover how incredibly different they sounded.
For the record: Japanese has a more fluent rhythm and a phonology that makes it sound like a mix between Spanish, Italian, and Finnish, more or less. Chinese is more sing-songy with heavier syllables. Also, syntax-wise they are very different, with Japanese using subject-object-verb and Chinese using subject-verb-object (with some odd caveats).
They are somewhat similar. Enough so that Japanese can pick out words. You wouldn't get the same result if you asked a Japanese speaker to translate sentences from Russian or French. Those are completely dissimilar. YouTuber "that Japanese man Yuta" asked Japanese people on the street, if they can read Chinese sentences (Cantonese) , he is trying to do a separate video for Mandarin. https://youtu.be/-E6vHCT0wpw