How Do I Do Multiple Things at Once?

Arduino runs one loop. The trick is to avoid delay and use separate timers for each task.

Basic pattern

multiple_tasks.ino

cpp

unsigned long ledTimer = 0;
unsigned long sensorTimer = 0;

void loop() {
  if (millis() - ledTimer >= 500) {
    ledTimer = millis();
    // toggle LED
  }

  if (millis() - sensorTimer >= 100) {
    sensorTimer = millis();
    // read sensor
  }
}

Each timer is independent, so tasks run on their own schedule.

Bottom line

Use one timer per task and avoid delay.