I am using Gsmarena photo camera comparasion tool and I compare iPhone 6s plus and iPhone X in low light. Why iPhone 6s plus looks much better and less noisy compared to iPhone X while iPhone X has much better sensor? Or am I wrong about image quality criteria in low light?
Peering at the pixels of these images really isn't going to tell you a whole lot. They both show a lot of noise. It looks like the first image has more noise reduction applied: it appears to have fewer speckles ("noise"), but also seems "smeared". The lower image has more apparent noise, but also sharper lines and contrast. There is probably more actual detail — but it's hard to tell because the scene doesn't contain much fine detail to peep at.
I have a hard time saying which is objectively "better", and although I have a personal preference for the lower one if this is all I had to choose from, if I really wanted a picture of some colored pencils, I'd get some more light and all of this would be irrelevant.
I think it's more a matter of algorithms in each of these phones slightly differing. Both appear to be low quality in the shadows. The iPhone X seems to over-sharpen the result and the iPhone 6s Plus seems to do some automatic noise reduction (or at least not perform as aggressive sharpening). As others have suggested, it's hard to know what "good" is in this scene without a better reference. Maybe taking a photo of the same subject in the same light with a "full frame" DSLR would allow us to see enough of the actual detail to make a judgement about which of the phone cameras renders the image better.