How 3d anime games could be made with unity?

Yes, it can be achieved. Take a few dozen millions of dollars, assign it to a professional art team numbering in the dozens, and watch the results. Seriously, there is a reason why these titles have such good graphics; they have the budget to pay highly qualified artists plus art directors, shader programmers etc.

If you have the skills you can of course do it yourself (how long that takes is another matter), and if you don't learn it. It's no witchcraft but there's a reason good artists commonly have years of practice under their belts.

I mean, Unity-chan is a free example of this sort of art and has been downloaded thousands of times. As for the world itself, it's surprisingly easy to make cartoonish grass in Unity using the vanilla terrain system and several free assets on the assetstore to help get you started.

Without much experience, you can still get good cartoon results with Toony Colors Pro 2 asset: https://assetstore.unity.com/packages/vfx/shaders/toony-colors-pro-2-8105 There are lots of samples that you can test on their website.

There's also a free version with a few basic styles: Toony Colors Free https://assetstore.unity.com/packages/vfx/shaders/toony-colors-free-3926