If you’ve ever wondered how Mozart et al were so productive, they were trained in a very intensive way. They learned the rules for counterpoint etc as rules for improvisation. In other words, they would have a large number of memorized melodies, such as gregorian chants. They would slowly learn more complex rules for combining two melodies so that they could slowly improvise more and more complex counterpoints on top of the melodies they already knew. In addition, they would memorize large numbers of stock resolutions and cadences to stretch out pieces.

So they would build up to writing symphonies in a very structured way. First improvise a melody. Then stretch it out using stock cadences into a dance. Combine several of these together into a symphony. This is only the barest introduction, but in general musical genius was taught with less focus on individuality and self direction. They attempted to teach improvisation, composition, and genius in a very structured and progressively graded manner.