"Writing" was done by manually stringing wires though those big rings. Those could be changed, but it was a big headache.
It's like classic reverse DNS. There was already a mapping from A to B (phone number to outgoing wires) and a mapping from B to A was needed for billing purposes. But B to A info couldn't be obtained from the switch fabric. A physically separate B to A mapping had to be built and maintained in sync. There was nothing which inherently made the two match. That was all done by hand.
The early history of computing was a struggle to find a usable memory device. Relays were very bulky as memory devices, and none of the relay computers had much memory.
It's like classic reverse DNS. There was already a mapping from A to B (phone number to outgoing wires) and a mapping from B to A was needed for billing purposes. But B to A info couldn't be obtained from the switch fabric. A physically separate B to A mapping had to be built and maintained in sync. There was nothing which inherently made the two match. That was all done by hand.
The early history of computing was a struggle to find a usable memory device. Relays were very bulky as memory devices, and none of the relay computers had much memory.