In very basic terms the ball follows the laws of physics, fluid dynamics and aerodynamics. Once the ball leaves the pitcher's hand, science takes over. Gravity is certainly something that will make a pitch do something other than go in a straight line. The other thing that makes a ball move is an air pressure differential on one side of the ball compared to the opposite side. If an area of high air pressure forms, the ball will be pushed away from that. If an area of low pressure develops the ball will move into it, being pushed by the relatively higher air pressure elsewhere around the ball.
Softball pitchers take advantage of each of these two effects. The one that creates a pocket of high air pressure is called the Magnus effect. It was first noted by Isaac Newton deciphering what made tennis balls curve. Heinrich Magnus figured out the many things that go into how much a ball would break, hence the name associated with that effect.
But it was clear that the ball did not always follow the path predicted by the Magnus effect. It wasn't until 2019 that Barton Smith and his team determined what made a baseball slider break seeming unexpectedly compared the gravity and Magnus force alone. It was named the Seam Shifted Wake (SSW) effect. Softball pitchers had taken advantage of SSW pitches for many years before the phenomenon was discovered. But now that it has been identified pitchers can proactively design pitches to take advantage of it rather than just allowing it to happen.
These two effects are described below. How to make the ball move using either of these, or both on the same pitch, is also described.