How does Serial.readString() manage the serial buffer?

Processing Electronics (Arduino, etc.)
2

there is a wide range:
from read byte

  if ( myPort.available() > 0) {  // If data is available,
    val = myPort.read();         // read it and store it in val
  }

to handle full lines

void serialEvent(Serial myPort) {    // if want to see what the arduino is answering:
  String arduinoline = myPort.readStringUntil(10);
  if ( arduinoline  != null )  print(arduinoline);
}

https://processing.org/reference/libraries/serial/Serial_readStringUntil_.html
https://processing.org/reference/libraries/serial/serialEvent_.html