what is the most demanded work in cg trade and demanded product?

Really ? And why would anyone wanted to share that information with you when they can make that model/s by themselves and earn money instead of you ? Why would anyone wanted to create competition to itself ?

Buyers have a demand for good products at good prices, created by designers who, instead of asking such questions, regular practice and improve their skills.

Million dollar question........Use the Analytics to find out what people are searching the site for! Also this may help you https://www.youtube.com/watch?v=xI_HpXuPWms&t=71s

Make evergreen content that is always relevant. These are characters, plants, rocks, natural landscapes, futurism and fantasy. It is clear that you need to make models of decent quality and set an adequate price. I do not advise you to deal with furniture, jewelry, cars (these categories are crowded), weapons (also crowded, and there are also restrictions on marketplaces due to the possibilities of 3D printing). Don't chase trends because they don't last long