Problem with exporting (blur/pixels)

Hi, i have got issue with blur/pixels in my rendered video (care about audio, its loud in one moment). Youtube has made it even worse, but even in the rendered video on my computer there are also pixels and this blurry stuff. You can see this clearly in 0:21, in the black background. In the source video there is no such thing, it's all in a very good quality. I'm using Davinci Resolve 16, i alredy set my bitrate at 46000 (same as source video), but still quality of this video is really low. Can somebody help me and tell why?

Too dark and too little detail in those areas. Poison for compressors.