Qwiic is SparkFun's I2C prototyping system, it's only for use with I2C devices. There's also only one cable, a JST SH 4-pin connector and will only use it for I2C. Qwiic is a bit newer than Grove, it was created around May 2017.
- Like Qwiic, STEMMA only uses the 4-pin connectors for I2C. The STEMMA QT connector is identical to the Qwiic connector and uses the same pin ordering.
- Qwiic only has level shifting and voltage regulation on the controller, not devices. So, you can use STEMMA + STEMMA QT devices with any Qwiic controller and you can use Qwiic devices on a STEMMA controller if you set the voltage jumper from 5V to 3V (and have a cable that converts the 2mm to 1mm pitch connector)
This company also makes sensors and breakouts with the Qwiic connector, it is the same voltage as Qwiic so you can treat it the same