Where can I find the firmware of EFM/ATSAMD21G16B-AU

just find the similar thread Does anyone have the firmware of ATSAMD21G16B on Jetson_AGX_Orin_DevKit_Carrier_Board, if ATSAMD21G16B-AU is only used for USB to serial port debugging, then this firmware is not needed. We will adopt our own debugging scheme for debug purpse