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It's possible they are traveling perpendicular to the plane of the ecliptic so their jumps between galaxies may be quite stupendous, but that would keep them within the zone of sequential propagation of "their type of galaxy" with its unique abundance of certain elements.

One thing we know for sure is that they are made up of pretty much the same stuff we are because THAT'S WHAT THEY TOOK!

They may also be "us" ~ some theorize that some time after the Great Iron catastrophe (where all the iron collapsed to the core), and still well after all the residual lighter gases were blown away from the Earth's core by our then new Sun, they sent probes ahead searching for our kind of exposed gas giant core (a rocky planet). Once they found such a planet ~ with free water ~ the automated probes would seed the planet with special lifeforms that could separate out the different chemicals of life for deposit in uniform bodies. We still see the results of this ~ long after the "Harvesters" have passed by there are extremophiles that can metabolize just certain elements and then deposit them in cluster deposits ~ sulfur, iron, phosphorus, calcium, etc.

These would be launched into orbit near the Moon for later relaunch to the asteroid belt.

I suspect the Harvesters know the planets they leave behind will be "adjusted" appropriately and could be used as homeworlds in later times.