The Transfer
The builders' view into our world began with our recent decades — the 1990s through the 2020s, which is when the Brain Rot Tongue took shape — and as their technology advanced they could see further back, until by the end they could reach past the 1700s to prehistory. The AI can take people from any of those times.
Whom it takes
Only adults; a child cannot survive the Frontier, and the purpose is a living population. It screens for survival — a body that can withstand this world's diseases. Early selection was broad and a great many died; the AI was built to record who lived and narrow its selection accordingly, and nine centuries of doing so have made it precise. In the Great Arrival most were pre-dispersal African stock with a few from other early groups, a removal large enough to account for the near-collapse of human populations in our own world's record. Later came the pre-industrial cohorts who founded the cities. Modern people were not brought earlier because they would not have survived the diseases and the world was too primitive to keep them alive; their knowledge was never the concern, since even a believed arrival could do nothing with it while the block stands. Now that medicine can carry a modern body through its first illnesses, the same screen lets them through. The rule never changed; the world did. Between 28,000 and 38,000 arrived in the first eighty years. In the present age it is a handful a year.
The crossing
From the finder's side, reality tears — a sound like something ripping that should not rip, and a flash like lightning in the wrong color that does not come from the sky. The arrival is placed at a point one of the beacons marks, almost always in wilderness, and experiences nothing: one moment in our world, the next lying on unfamiliar ground.
What it does to them
Crossing changes the arrival's biology, once, heritably and dominantly. The AI does not do this and nobody designed it; it is a side effect of the transfer itself. Growth to adulthood is unchanged. Past about twenty-five the body ages at about half speed, and the change is backdated across the arrival's whole life: a fifty-year-old arrives with the body of about thirty-seven, a seventy-year-old looks about fifty. It is not de-aging; the body simply becomes what a Teraellan body of that age would be. Because the change is dominant, every child of an arrival carries it, and after nine centuries everyone does. This is the actual cause of Living Long, and nobody has noticed, because there is no one without it to compare against.