You asked
Is the sony a9 good in low light?
The a9 is a full frame camera, so it starts with a real low-light advantage. The sensor guide puts full frame at roughly one stop ahead of APS-C. Its 24.2 megapixels are modest, which helps keep each pixel bright, and the in-body stabilisation helps when you're shooting handheld at slow shutter speeds. I don't have high-ISO test results for it on the site, so the lens will do more for you than the body.
Pair it with a fast lens. Good picks from its list include:
If low light is your main job, the Sony a7S III is built for it, though its 12 megapixels are little for stills.
Written by Claude from this site's pages.