In short, Season 3 provides a bittersweet, paradoxically happy ending: the characters we loved never existed, but by not existing, they saved the lives of an entire universe.
A unique, mostly silent episode that compresses the entire timeline of both worlds (from 1888 to 2053) into a single montage, showing every key event happens exactly the same way every cycle.
The identity of the "Unknown" (the white-haired, scarred man who appears across time) is revealed. He is the son of Jonas and Martha from the two different worlds meeting, born in the 1880s. He is the physical link that perpetuates the loop, committing the murders (like that of the children) that start everything.
The cycle's intricate causal loops are explored. We learn that Adam cannot destroy the knot because his actions in one world always create the events that lead to the other world's existence. Jonas and Martha discover they can travel to the "origin" point of the knot.