If you do everything correctly and there really is a lot of noise in good light at low ISOs, you need to have the camera replaced by the seller. There's something wrong with the camera.
But it would probably be a good idea to get user error ruled out first. Any way you could send me a raw file with a shot showing the problem? Shot in good light at ISO 100. I'll be happy to take a look and tell you if it looks like the quality out of my A7III.
