To turn your starter, electricity flows from the battery, through the ignition switch, and to the small terminals on the starter solenoid. This electricity causes an electromagnet inside the starter solenoid to connect the two heavy gauge wires and form a complete circuit. High amperage electric flow then goes through your starter, turning your engine’s flywheel.
This article serves as a basic introduction to what is possible with hydraulics for your garden tractor. There are a wide variety of pumps, valves, motors, hoses, etc. I hope this article inspires you to research more of what you can do with the physics of fluid.