Subjects of Ymir and pure titans
The Subjects of Ymir are Eldians, that is, those who live in Paradis and the honorary Marleyans living in Marley. 
The injection will only work on Subjects of Ymir. Injecting other humans would have no effect on them.
The story presented in the manga and anime by Marleyans was that Ymir acquired it by making a deal with the devil. Another version of the story was also presented.



  According to Eren Kruger, this was done when she came into contact with the "source of all living matter" as a young girl. With this power, Ymir became the progenitor of all Titans


No additional information was ever presented. We'll have to wait for additional chapters from the mangaka.