Hello,
receivedChars is used since it stores the values to send through wireless communication. Please note that the Arduino receives a message from the Serial Port and this message is stored in receivedChars.
I'm checking out for readBytesUntil() and keeping you up to date.
For the Tx, both println() and write() give the same result, since I'm not printing the message directly on the port but rather reading the Serial Port with an alternative Python code. Whether it is printed or just send to the Serial Port doesn't change anything (or at least it seems so when running).