You asked
Is the sony a7r vi good in low light?
Here is what the site says about that.
From Sony a7R VI: Sony a7R VI: A mirrorless camera by Sony with a 66.8-megapixel full frame sensor, announced in 2026. Sony's highest-resolution body, with a fully stacked sensor that shoots at 30 frames per second. Specs - Maker: Sony - Mount: Sony E - Sensor: Full frame, 36 x 24mm, crop factor 1.0 - Resolution: 66.8 megapixels - Stabilisation: in the body - Announced: 2026 - Type: mirrorless camera Focal lengths on it Full frame is ...
From Sensor sizes: Is full frame worth it?: For low light, events, and portraits with very shallow depth of field, yes. For travel, street, wildlife on a budget, and most hobby photography, a good APS-C camera with good lenses is enough, and you'll carry it more. Lenses and light matter more than the last stop of sensor performance.
Quoted from this site's pages.