For the 2.4 GHz part, the nRF is a transceiver meaning it does both TX and RX.
For the 433 mhz part, the PCB referenced in the first post has an "Aurel RTX-MID" which is a transceiver and thus both TX and RX so you can receive RF signals from 433mhz devices and also send commands via RF to 433mhz switches, doorbells etc.
The software provides state switching signals to control the transceiver mode if one is used.
Of course you do not need to use a transceiver, seperate TX and RX modules works just as well.
You can use almost any a separate receiver and transmitter module like mentioned before..
For sending you can use a cheap Chinese transmitter (like the FS1000A XY-FST from the XY-M5-5V set) which can be nice as many people have these lying around..
The receiver from such a set is in most cases pretty poor because it receives a lot of noise.
(Btw, these Chinese sets seem to perform a little bit better at higher voltages.)
