"The commission" is the "trusted third party mentioned above” In the case of the Facebook partnership, the commission is composed of (a) Gary King and Nathaniel Persily as co-chairs, acting like editors of a scholarly journal and (b) other members, acting like members of the journal's editorial board, but organized into methodological, substantive and regional committees. Different subsets of members (as needed, cross-cutting the committee structure) sign one of three levels of confidentiality agreement, with only the most restrictive of which has privileged access to proprietary information at Facebook and, as a result, foregoes grant applications and publication without pre-approval.
The commission is a unified structure and so it (although not every one of its members) has full access to the necessary datasets and information at Facebook needed to conduct research. The decision-making structure ensures that "the commission" can access whatever information it needs without unduly burdening its members or creating a security risk by having every single member access proprietary or privileged information that they don't necessarily need for a particular decision or analysis.