You’ll upload/send the ESP32 code through the USB port on the ESP32 board/module using the Arduino IDE, PlatformIO, or ESP-IDF.
For this project, the usual flow is:
- Connect the ESP32 board to your computer with USB.
- Open the code in Arduino IDE or PlatformIO.
- Select the correct ESP32 board and COM/serial port.
- Click Upload.
- If upload fails, hold the BOOT button while uploading, then release it.
If you mean “where should I put the code in Flux,” then you can add it as a project file/document for reference, but the actual firmware is uploaded to the ESP32 from your computer via USB.