On the BQ25120A, both the /INT (Status Output) pin and /PG (Power Good) pins are an open-drain outputs, you can connect these pins via a pull up resistor or LED to the desired logic voltage. If monitoring the /INT or /PG pins via a microcontroller (MCU) application, you want your digital level to match by tying it to your MCU digital power rail.
On the I2C BUS (SDA/SCL) you need to connect your I2C pull-up resistors to the VSYS rail. Keep in mind that the I2C Bus of your system is shared between slaves and needs to match the logic level of your MCU.