I couldn’t edit my previous reply to my own post, so deleted it and am posting a new one here.
After some trial, I found I had installed wrong (not sure why exactly, though) board in the Arduino IDE. It should be ‘Seeed nRF52 mbed-enabled Boards,’ not ‘Seeed nRF52 Boards.’
Once you install this board correctly, the variant.cpp (in the folder /.arduino15/packages/Seeeduino/hardware/mbed/2.7.2/variants/SEEED_XIAO_NRF52840_SENSE/) lists all the pin definition. Here is a part of the file for the LED pins.
// LEDs
{ P0_26, NULL, NULL, NULL }, // D11/LED RED
{ P0_30, NULL, NULL, NULL }, // D12/LED GREEN
{ P0_6, NULL, NULL, NULL }, // D13/LED BLUE
In the same folder, pins_arduino.h also shows pin definitions.