Why is rbAmp 5 V-powered but the I²C bus is 3.3 V?

The module has its own microcontroller doing the measurement, and it needs clean, stable power to keep the ADC readings accurate. So the module takes 5 V on VCC and runs it through an on-board low-noise regulator that derives the 3.3 V its MCU actually runs on. Because the MCU is a 3.3 V part, the I²C lines (SDA/SCL) operate at 3.3 V logic — they're 5 V-tolerant, but they idle at the host's 3.3 V level. In short: power the module at 5 V (4.5–5.5 V; below 4.5 V the ADC loses accuracy), wire the I²C bus at 3.3 V logic. On an ESP32 or classic Arduino that's automatic — the bus follows the host's 3.3 V rail, independent of the 5 V supply.