Safety disclaimer: this is a thought experiment, some actions which I describe are dangerous in reality, do not experiment with electricity if you’re unsure that you know what you’re doing.

Take the words earth and ground literally, it’s really a thick metal rod which gets hammered into the ground when a house gets built.

A voltage source like a battery or a generator creates a difference in electric potential between its poles (by separating charge).

Think of hills and valleys separated by a solid wall on isolated islands floating in the sky for some reason… 😅 The islands are of arbitrary height and free floating, but their height difference is fixed. On each of them you could do the ball on a hill experiment and it would yield the same result.

You can connect the hills and valleys of each island by attaching a highway with its own wall. This is the equivalent of connecting the poles via a resistor (lower electric resistance means more and bigger holes in the wall, hills/valleys will deplete faster).

You could also connect these islands in a way so that the valley of one and hill of the other touch. If you use low resistance wires there is nothing stopping electrons to freely flow from one island to another, whatever height difference there initially was between the valley and hill of the two islands, instantly drops and the islands get pulled together. But no electrons actually flow (no height difference is driving them), no hill and valley of any individual island gets depleted, no energy transferred - that’s essentially what chaining 9V batteries to yield multiples of 9V is.

If you ground one pole of a generator, you pull the island toward the earths surface, which in itself can be considered a flat, but practically infinite valley/hill. Earth acts as a kind of mass storage for electrons not changing its height much when letting charged particles flow into it or vice versa.

So if you had two grounded generators:

• one with 200V height difference, grounded but using a highway with a wall with few holes in it (poor ground connection)
• one with 100V with a highway with a wall with few small holes As long as nothing is depleting these islands, their valley will be fixed at ground level.

If you were to connect the hill of the 200V generator to the valley of the 100V generator, the valley and hill of the 200V generator would be directly connected by the ground resistor (and its own ground resistor but that can be neglected in this example), pulling the ground pin of the 100V generator to 200V - and that’s the answer to your question. In theory every grounding point isn’t actually a grounding point, but we do our best to keep resistance as low as possible so that we can safely touch metal objects in our houses.

Oh and „instant depletion transferring energy“ essentially means short circuiting a voltage source, so do not experiment with this, even if it’s low voltage like 9V batteries, they can start a fire too and in practice there are also chemical hazards. In the generator example a fuse would blow if there was one, stopping the depletion and preventing a fire or a molten generator.