it's all open source, you can see part of it here https://github.com/arduino/ArduinoCore-renesas https://github.com/arduino/ArduinoCore-renesas you can use GCC

it's all open source, you can see part of it here https://github.com/arduino/ArduinoCore-renesas https://github.com/arduino/ArduinoCore-renesas you can use GCC with https://github.com/renesas/fsp https://github.com/renesas/fsp if you want to go very low level.