Wiring of external IMU might differ. So far, I have tested both LSM6DS3 and BMI160. BMI160 offered less noise. This is based on the Flipsky 75100 so might differ for other ESCs.
First of all - it may be hard to distinguish between the two - they might look very similar - LSM6DS3 at the top, BMI 160 at the bottom.
For LSM6DS3 MII wiring is:
IMU GND - ESC GND
IMU 3.3 - ESC 3.3
IMU SCL - ESC SCL/TX
IMU SDA - ESC SDA/RX
For BMI 160 wiring is:
IMU GND - ESC GND
IMU VIN - ESC 3.3
IMU SCL - ESC SCL/TX
IMU SDA - ESC SDA/RX
IMU SAO - IMU GND
Some additional info: BMI160 module for DIY balanced board DIY onewheel with VESC | Spintend
External IMU setup is covered nicely in IMU (gyro) Configuration Guide
