Attack On Titan season 4 reveals how the Titans are really made - and it's a much sadder story than you might think. When Attack on Titan begins, the titular monsters are nothing more than flesh-hungry menaces, but as Eren Jaeger and his friends uncover the Titans' secrets, that perspective shifts. The power of the Titans was initially obtained by a slave called Ymir in the distant past. Following Ymir's death, this gift was shared between nine Titan-shifters, who can transform between their regular and Titan forms at will. The mindless creatures plaguing mankind throughout Attack on Titan's early episodes are known as "Pure Titans" - transformed humans who lose their intelligence and cannot change back.
Until now, Attack on Titan has presented its Titan transformations as an ability passed down through Ymir's Eldian bloodline. Only Eldians can become Titans (shifter or otherwise), and the Nine are transferred by eating the current owner. Should a Titan-shifter die without being chewed upon, their power shall be reborn elsewhere in another "Subject of Ymir". By Attack on Titan season 4's second half, these rules are widely known, but when Eren and Zeke Jaeger finally make physical contact in "Two Brothers," they're transported to the Paths to meet their fabled ancestor, Ymir, in person.
Face-to-face with the progenitor of all Titans, Eren and Zeke learn the hidden truth behind how Titans are made. Essentially, Ymir constructs them out of sand and "sends" the bodies to wherever in time and space they're required. Despite perishing many centuries prior, Ymir remains the true source of all Titan powers. She now dwells on a spiritual plane known as the Paths, where Titans past, present and future are all connected, allowing her to influence each and every Subject of Ymir.
As an example, Reiner Braun possesses the Armored Titan in Attack on Titan's present day. Whenever Reiner triggers his "transformation," he's actually calling upon Ymir. In the Paths, she constructs the Armored Titan's body from scratch using the realm's mysterious sand alongside a small pail of water, and that body is then sent via the Paths to wherever and whenever Reiner is calling for it. Because the Paths exist separately from time, Ymir can spend years making Reiner's Titan, but the transformation will appear instantaneous to those in the real world, including the Armor himself (though she must despair whenever Armin or Bertholdt's voices echo through the Paths).
The same process occurs whenever a Titan is hurt. If Reiner's eye was poked out by Mikasa's blade during battle, Ymir would create another eye in the Paths, then send it through to him, giving an impression of the Armored Titan healing itself.
With a 13-year lifespan imposed upon anyone who inherits a Titan, the Subjects of Ymir are already bound by tragedy. But discovering the crux of their power is a young girl previously enslaved by a greedy king, who now toils away creating massive bodies out of sand and lacks the will to refuse, makes Attack on Titan's premise infinitely more heartbreaking. There's a bitter irony to how Eren's fight for freedom derives from a girl who hasn't known the taste of liberty since she was born, and continues to serve obediently even in death.
Attack on Titan season 4 streams Sundays on Funimation, Crunchyroll, and Hulu.