After Ymir was captured, she admitted she was indeed Ymir under the expectant eyes of many believers, hoping to save them.
But the Marley Government didn't care about them at all, they didn't even bother with a trial, and directly injected Ymir and those believers with Giant serum, turning them into Mindless Titans.
This is the punishment for Eldian People after committing crimes. Becoming a Mindless Titan is worse than death; they have self-awareness but cannot control the Titan body, wandering like a spirit.
The Mindless Titans created from the Eldian People were not wasted by the Marley Empire. They were all transported to the island where the walls are located.
This is why there are endless Titans outside the walls.
After transforming into a Titan, Ymir wandered for a full sixty years. By chance, she devoured a companion of Reiner and others, gaining the 'Jaw Titan's Power' and regaining her human form. At that time, Ymir still looked like a teenager.
