There’s a tutorial on using the Xiao ESP32-C6 on PlatformIO in the Wiki. However there is another approach that some might find easier: pioarduino which is a fork of platform-espressif32 by Jason2866. Just use
platform = https://github.com/pioarduino/platform-espressif32.git#develop
board = seeed_xiao_esp32c6
in platformio.ini.
All the sketches in sigmdel/xiao_esp32c6_sketches can be compiled with the develop branch of the pioarduino fork. They can also be compiled with the stable branch if the seeed_xiao_esp32c6.json board definition file is added in a local directory.
Many thanks to Jason2866 for all that work.