It is optiboot — the 512-byte arduino bootloader — modified to support flashing over the nRF24L01+ ... but i could not find the schematic of connections from the module to the ‘mega328 chip.

[Ralph] wasn’t satisfied with the required 5 control pins to drive his nrf24l01+ transceiver module, so he used this circuit needing just 3 pin using an ATtiny85. One of the key components wa… ...

The NRF24L01+ Arduino wireless transceiver shield is ideal for a wide range of applications. For instance, it can be used in home automation projects to control lights, fans, and other appliances ...