Open Collector Technique
The output of an open collector (OC) logic gate in an integrated circuit is a transistor whose collector is connected to the output pin and the emitter to ground (GND). This output transistor can be in two states. It can be in a conduction state, so the output is connected to ground. In the second state, the transistor does not conduct current, so the output remains open (in a high impedance state between the collector and emitter).
Below in the diagram, I replaced the transistors with classic switches. Switch open, the transistor does not conduct (is in a high impedance state). Switch closed, the transistor conducts current.