I found the datasheet for the I2C chip commited as part of the python library. https://github.com/pimoroni/sn3218/blob/master/datasheets/sn3218-datasheet.pdf
Looks like they are using a 400kHz clock frequency, but I couldn’t find what the default frequency is for the circuits_i2c library.
The datasheet doesn’t document the I2C commands, but I did find an arduino library which is looks like it has a pretty straightforward API that mimics the I2C interface https://github.com/pimoroni/pimoroni_arduino_sn3218/blob/master/sn3218.cpp
