I'm looking to purchase thunderboard BG22, but i have a few question
1. how difficult is it to implement i2c sensor, I was only able to one sample code (from silicon lab github). From the look of it, i2c implementation is a little different from using Wire.h.
2. does the thunderboard app show external sensor data or just built-in sensor data on the board
1. Yes it's different from wire.h which is an Arduino library. To operate the I2C on the BG22 you can use emlib (https://docs.silabs.com/mcu/latest/efr32bg22/group-I2C) or a higher level driver (https://docs.silabs.com/mcu/latest/efr32bg22/group-I2C). As for examples it seems that you have already found them. In the Bluetooth SDK you can also look at the soc-thermometer example which uses I2C to read the temperature data from the Si7021 sensor on the WSTK.
Can't comment on how difficult it is, that is totally relative.
2. Just the built-in sensor, but you can expand the app yourself, the source code is available on github.