I agree in part but disagree in part on the multicultural counter - but agree JingWei is spot-on the other comments.
China is both homogeneous and at the same time, heterogeneous. Depends on what you see and what you're trained to see. Most foreigners view China and Chinese as a rather homogenous single culture and yet the cultural habits, diets, speech patterns across the length and breadth of China are as vast and myriad as perhaps even Western Europe. Within the Han race (the predominant race in China) - we are multicultural. Hong Kong, Macau, Taiwan, Guangdong, Guangxi, Anhui, Yunnan, etc ad infinitum - we have dialectic, dietary, and obvious behavioral differences - both in personal and professional nuances, not to mention the not too subtle differences in clothing and accessory choices - and I haven't even touched on the oh-so-obvious religious and philosophical institutions - Islam (guys with the white skull caps), the myriad flavors of Buddhism, Dao, Kongzi (Confucius) - also etc ad infinitum.
To the uninitiated - all chinese (or all asians) look alike. The clothing and accessory behaviors of Shanghai dwellers versus Beijing or Yunnan can either be utterly obvious or oblivious. A Beijing taxi driver can identify his client's nationality based on their haircut and clothing.
Chinese can identify other Chinese as non-local based upon the simple dialectic nuances of a greeting - "你好" (ni hao).
To me - a foreign-born Chinese - China, Japan, Korea are as obviously multicultural with the associated social complications as a European living in London might observe.
Each province is distinct and unique. Someone on the Chinese internet once posted an amusing cartographic summary of each province's famous eccentricities - some not too flattering, others merely amusing.
My unsolicited advice to jules77 - as a photographer - was to look for those subtle nuances and try to capture them in history. Ethnic differences are easy - just photograph the different costumes. Any tourist with a camera can do that - although a professional photographer can obviously make the subject matter so much more appealing, alluring, or entertaining - but to capture the subtle nuances between provinces, cities, rural areas, age groups, gender groups, and the infinitely diverse social cliques - that takes a sharp eye, a sharp mind, and some serious talent with a lens.