Links

Visualisation example

Arduino state machines
https://arduino.stackexchange.com/questions/63488/wanted-to-control-2-servos-with-serial-read/63491#63491
https://www.teachmemicro.com/arduino-state-machine-tutorial/

https://arduinoplusplus.wordpress.com/2019/07/06/finite-state-machine-programming-basics-part-1/

Example Arduino

As example of the general structure:

int state = 0;

void loop(){
    switch(state){
        case 0:
            if(Serial.available()>0){
                int input = Serial.read();
                //take appropriate action
                state = 1; //do state transistion
            }
            break;
        case 1:
            if(Serial.available()>0){
                int input = Serial.read();
                //take appropriate action
                state = 0; //do state transition
            }
            break;
    }
}