Are there actually any muscles in the throat that are trained and become stronger the more we sing or do we just learn how to co-ordinate them better? I'm curious even though it almost certaintly makes no difference to how one practices.
We just learn to coordinate them better.
If any muscle or group of muscles is likely to affect singing strength they would be at the walls of the thoracic cavity. So thats diaphragm and exterior intercostal (side) muscles for inhaling and abdominal and interior intercostal muscles for exhaling.