In the following video from Kenny Cardoso (Kardoso Media) shows just how well the Sony a6400 together with the Sigma 30mm F1.4 DC DN handles shooting video in low light.
The video is a compilation of low light footage that he has captured over the last 6 months.
All footage was shot in 1080p 120fps with ISO ranging from 100-800 at f1.4 and in picture profile PP1. No colour correction or colour grading done. He also says that you can expect even better results if filmed in 4k and in lower frame rates.