Repeated physical actions eventually lead to habits. An action becomes a habit when it is repeatable with less and less conscious attention. Many of the singers who seem to sing “naturally” got there by painstaking work and practice. What is natural now may have felt very unnatural in the beginning!
When complex or strange tasks become easy over time with proper repetition, we sometimes say that they become “second nature”. I like that term because it implies there was a “first nature”. In the beginning, the natural way to do something new can be very awkward and even dangerous. For example, loud or high singing by someone who is unskilled can be painful for both the singer and the listener. But with intelligent work and steady practice, a new natural is created.
My teacher who said “there is nothing natural about classical singing” was saying that most people need extensive (re)training in order to master the art. When we get to repeatable, easy singing, we can make the audience and even ourselves believe that we have found a “second nature”. That’s a great accomplishment. It’s worth the work.