I've been down this road before. The ASIC in the Milwaukee pack is known as an "analog front end". Since you can't wire cells directly up to the pins of your MCU, you have a chip to handle that stuff for you. It will let you read the voltage of each cell in the stack (some are literally analog, and just provide a nicely in-range voltage output for your MCU's ADC to read; others have internal ADCs). It might do current sensing. It might handle a couple alarms to turn off a mosfet when things go wrong. It will have an I2C interface, but the supported commands are "tell me voltage of cell #2" or "put shunt resistor on cell #1 for balancing". It's still the MCU that calls most of the shots here, but the more advanced ASICs can do more things autonomously.