What the history behind music?

Answer 1

In the west:

Pythagoras discovered that simple ratios between frequencies sound consonant and developed the equal temperament scale thousands of years ago.

For many years this scale was abandoned for church and folk scales, but in the 15/1600s composers adopted the scale and it is still in use.

There were strict rules.

These rules have relaxed allowing composers to write more expressive music, until in the early 20th century , Schoenberg broke away from Pythagoras' system completely.

Since then, composers have been trying to develop new systems.