Does it help you in learning Japanese.
I actually learned Japanese first, but yes, my knowledge of Japanese kanji has been a big help in learning Chinese kanji. When I first started learning Chinese I knew about 500 Japanese kanji, and it helped me a lot to learn Chinese kanji.
By the way, in Chinese, certain kanji are used only for their sound, in a way that Japanese kana are only used for their sound. For example, the Chinese kanji 尼 is used for the sound "ní", most Chinese people use it for the sound only, and are not aware it means "Buddhist nun". (But just by looking at Chinese, you cannot tell which kanji are used for meaning and which are used only for sound.)
Those days when Japan took the Chinese characters over I believe China hadn't unified the characters and designated an official language yet.
Regarding written kanji, they have evolved for thousands of years, and finally reached their final forms (the way they are written) about a thousand years ago.
Regarding spoken Chinese, there are over thousand dialects of Chinese! In addition…
"Some form of Mandarin has served as a national lingua franca since the 14th century. In the early 20th century, a standard form based on the Beijing dialect, with elements from other Mandarin dialects, was adopted as the national language."
en.wikipedia.org
But the only government which has effectively designated one national language has been the communist government. There have been about 24 Chinese dynasties, and some of them like the Ming Dynasty were huge, but none of them truly united China into one country with one official spoken language until the communist government came along. You may enjoy watching this animated video of the rise and fall of Chinese dynasties from 717 BC to today. (The video lasts seven minutes.)
Another thing is that vast majority of Chinese people do NOT speak Standard Mandarin, and school children must learn Mandarin as a foreign language in school. Because of this, lectures in Chinese universities are given in Mandarin, and college students have no problem communicating with their teachers and fellow students in Mandarin. (This was NOT true about 40 years ago.) Another thing about the large number of Chinese people who do not speak Standard Mandarin, TV shows and TV commercials always have Chinese character subtitles, so everyone can read them and understand what is going on.
Depending what Chinese dialect it was modeled on the resultant Japanese pronunciations were bound to be different from the modern-day Chinese.
My Japanese professor in college could actually tell you which part of China a pronunciation of a Japanese kanji came from and what year it came to Japan. (Chinese kanji were brought over by Chinese Buddhist missionaries, who were very successful in spreading Buddhism to Japan.) That was very impressive. One of the reasons Japanese kanji have so many pronunciations is because every time a new Chinese Buddhist missionary would come to Japan, he would speak in his own dialect, and Japanese people learned his dialectical pronunciation for a particular kanji, causing that kanji to have one more new pronunciation in Japanese.
Did you know the Japanese kanji 生 has something like 40 pronunciations, and there are something like 20 ways to say
I in Japanese?