The infrared transmit and receive sensors are located on either side of the RESET button and accelerometer at the center of the board marked TX (the IR LED transmitter) and RX (the IR receiver).
For MakeCode, there is a limitation to only sending a number via IR. There is more capability in CircuitPython. It is best to check both code example pages to get a feel what each language can do.
The MakeCode example assume there are two Circuit Playground Express boards.
The CircuitPython example uses one Circuit Playground Express and an Adafruit mini IR remote control.