Board Thread:Manga/@comment-34196931-20190808205129/@comment-27125793-20190812090445

So, basically, it's finally explained how the Paths and the Power of the Titans work, especially the Founding Titan's. Essentially, the wielder of the Founder doesn't truly have the power of Ymir herself; that power is still with Ymir, whose soul has remained inside the Coordinate. However, she is subservient to anyone who possesses royal blood, and they can use her to exercise the power she has in the ways they want. But that is only possible if they reach the Coordinate somehow and command her from there. That's what the Founding Titan is for; a medium that allows a royal to reach that place. If I have to use an analogy, and I apologize if it isn't particularly clear, I'd use the one of the man holding a loaded gun. The holder of the Founder is the man, the Founder is the gun, while Ymir (or more specifically, her powers) is the bullet.

It also explains how Zeke can use part of the Founding Titan's power; because it isn't REALLY its power. It is just a medium to access the Coordinate, only capable of being activated by a royal. Zeke, by holding the one of the Nine (who happen to hold remnants of Ymir's soul, and therefore can be argued to have a direct connection to her, but not necessarily to the Coordinate, as we saw when she repaired Zeke's body), and then injecting an Edlian with his spinal fluid, can create a direct connection between them and himself. Then, thanks to the Beast Titan he holds, he can (unknowingly to him, at the time) command Ymir to exercise her power through his directions on all the Eldians he has connected himself to.

Eren was able to use Ymir's powers by coming into contact with Dina, who was a mindless Titan. For him, both the Founder and Dina herself were the medium, but she functioned as a medium in two ways. The first is the direct link to the Coordinate, and the second was in allowing him command of Ymir by essentially fooling her into believing he himself was a royal. Dina was a Pure Titan, while he himself held two of the Nine, who are remnants of her soul, so it's his will in that case that came through, but since he reached the Coordinate, Ymir had believed that he must therefore be a royal.

Furthermore, it does appear that non-royal holders of the Nine (minus the Founder) can also exercise a comparatively miniscule amount of authority over her. This can be seen in their ability to transform into Titans in the first place when they wish it (plus injuring themselves; that part I don't get), as well as the ability to control their regenerative powers. Basically, when it comes to their own bodies and Titans, Ymir will act on their will, thanks to the connection they have with her.

In essence, non-royal shifters have power over Ymir, while royals with the Founder have power over both her and her control over the Coordinate that connects all Subjects of Ymir together.