How does this voltage detector IC work in a circuit? What does open drain vs. CMOS output mean?
An open drain output refers to an output stage where the pull-up transistor is missing. This means that rather than outputting a high voltage when the output is high, it presents a high impedance to the circuit and an external pull-up resistor is used to provide the high voltage. This is useful when there is more than one device that wants to output to the same line; since none of the devices can pull the line high, there is no way to create a high-to-low short circuit.

A CMOS output generally refers to a normal push-pull output where the low voltage is no more than 0.1VDD and the high output is no less than 0.9VDD.