Agency is not held, it is not a property of persons
or things; rather, agency is an enactment, a matter of possibilities for
reconfiguring entanglements. So agency is not about choice in any liberal
humanist sense; rather, it is about the possibilities and accountability
entailed in reconfiguring material-discursive apparatuses of bodily
production, including the boundary articulations and exclusions that are
marked by those practices. One of the items that you asked about is the
how of agency, and in a sense, the how is precisely in the specificity of the
particular practices, so I cannot give a general answer to that, but perhaps I
can say something helpful about the space of possibilities for agency.

(Barad, "Matter feels, converses, suffers, desires, yearns and remembers", 54)

First of all, agency is about response-ability, about the possibilities of mutual response, which is not to deny, but to attend to power imbalances. Agency is about possibilities for worldly re-configurings. So agency is not something possessed by humans, or non-humans for that matter. It is an enactment. And it enlists, if you will, “non-humans” as well as “humans.”
At the same time, I want to be clear that what I am
not talking about here is democratically distributing agency across an assemblage of humans and non-humans. Even though there are no agents per se, the notion of agency I am suggesting does not go against the crucial point of power imbalances. On
the contrary. The specificity of intra-actions speaks to the particularities of the power imbalances of the complexity of a field of forces. I know that some people are very nervous about not having agency localized in the human subject, but I think that is the first step—recognizing that there is not this
kind of localization or particular characterization of the human subject is the first step in taking account of power imbalances, not an undoing of it.

(Barad, "Matter feels, converses, suffers, desires, yearns and remembers", 55)