To process the sensor's data, you can use specialized libraries or manual interrupt-driven code. Flow sensor YF-S201 - General Guidance - Arduino Forum
volatile int pulseCount = 0; float flowRate = 0.0; unsigned int flowMilliLitres = 0; unsigned long totalMilliLitres = 0; unsigned long oldTime = 0;
if (millis() - lastTime >= 1000) lastTime = millis(); // The YF-S201 gives 450 pulses per liter flowRate = (pulseCount / 450.0) * 60; // Calculate flow rate in liters per minute pulseCount = 0; // Reset pulse count