On a semiconductor chip, what state (on/off or read/write) corresponds to conducting or insulating behavior?
In order to understand how physical background of the computers works, you should consider higher-level abstractions of the semiconductors. Otherwise, you will get stuck in a mess of semiconductors. Without basic knowledge of transistors and how these transistors form a logic gate, it will not be easy to comprehend the memory structures and read/write cycles.

To visualize, in CMOS technology, there are two complementary logic structures where one of them is connected to the ground and the other one is connnected to the high voltage source. In rest, these structures are "on" and "off" complementarily. When your output changes, on-off states of them are inverted, thus there are both 'on' and 'off' logic in every case. The scheme is completely different in another technology, let's say PTL.

In the memory case, 'HIGH' and 'LOW' voltages represent immediate memory state rather than individual 'conducting' and 'insulating' states of transistors. If a transistor between low voltage source and your memory output is 'conducting' while a transistor between high voltage source and your memory node is 'insulating', then you have a 'logic 0'. Still, all of them are technology dependent.