Via SPI. This is how its connected:
SD > Xiao
------------
3V3 > 3V3
CS > D3
MOSI > D10
CLK > D8
MISO > D9
GND > GND
Also, this is what the platformio.ini looks like:
[env:seeed_xiao]
platform = atmelsam
board = seeed_xiao
framework = arduino
lib_deps = greiman/SdFat@^2.2.2
upload_port = COM6