How AOT's main trio set Ymir free
Eren, Armin, and Mikasa all taught the Founder Ymir valuable lessons, and were instrumental in setting her free. All three of them caught Ymir's attention and she made the effort to reach out to them personally in different ways, observe them, learn from them.
First Eren reached her in the Paths and told her that she's her own person, and can choose what SHE wants rather than follow orders. This was the first time in her long, miserable life, that someone called her a human being. The first time in 2000 years (that would have felt even longer in the Paths, closer to millions of years), that she was told what happened to her was wrong. In her life, she had spent her whole life as a dehumanized slave without any acknowledgement of her own humanity. Eren opened her eyes (literally) to the injustice of her situation and taught her autonomy.
Then Armin arrived to fight Eren, and Ymir watched his every movement closely, even stealing him into the Paths with the Okapi and trapping him there. She watched his conversation with Zeke where he shared that the meaning of life is in appreciating the little moments and the love you already have rather than chasing an unattainable dream. In her life, she longed for an unattainable love/validation from King Fritz who was incapable of giving it to her, since he's a monstrous psychopath. She lived and died yearning for something she would never receive, rather than living for what she had (her daughters). Armin taught her the true meaning of life.
Lastly, Mikasa. From the moment Ymir noticed Mikasa's love for Eren, she was intrigued. She began to peak through Mikasa's lifetime memories, causing her headaches. Then Mikasa killed Eren despite loving him, knowing it's the right thing to do and proving she's not a slave. Ymir herself couldn't do this in her own life, instead choosing to die for the monster she loved, instead of killing him. Mikasa taught her that love does not equal submission.
In the end, all three of the main trio were instrumental in setting Ymir free. In her last appearance, Ymir had a vision of what she wished she had done, what she WOULD have done had she received these three lessons earlier in her life. She would have protected her beloved daughters, the truly important and precious things in her life, rather than being a slave to the abusive king. If she were given another chance, she would have let him die.
In the end, Ymir's deepest most primal desire was a pure and loving human connection, some semblance of tenderness and care, something she was so deprived of. This desire was so strong that it bled into every single Eldian, connecting them all to Ymir through the Paths. One lonely, abused little girl's desperate need to be loved, enveloping every one of her subjects, binding them all to her in an all-consuming web. She has always been desperately reaching out to other people.
In her ill young mind she mistook the king's sick """rewards""" as that love she craved. Eren made her realize "what he did to me was wrong and I have a right to my anger", Armin made her realize "I shouldn't have waited for love from him that would've never come and instead cherished the love of my children", Mikasa made her realize "I should have let him die/killed him myself". Acknowledging all this and making peace with her tragic life and the decisions she made, allowed her to let go. She finally moved on, and in this moment, she appeared not as an immature child, like she did throughout the Rumbling, but now as a grown adult woman.
Ymir's story in the finale is essentially a coming-of-age story from the perspective of a mentally ill, dehumanized slave. She starts the finale as someone with arrested development lashing out at the world, and ends it as an adult at peace with herself. Eren invited her to indulge in her rage. Armin helped her consider what was truly important. Mikasa healed her and allowed her to let go. All three protagonists were valuable and influential in freeing Ymir.
