It’s issued by the FAA based on the reports from the manufacturer, which can collude, falsify or mislead the authorities. The FAA does 0 testing on the airplanes, it is all done and documented by the manufacturer.
I'd guess you're thinking of the type certificate.
Delta has its own CAMO (Delta TechOps), which I assume has done the maintenance of this aircraft, but it is yet to be seen whether they followed everything to the T or is there something missing from Boeing’s part.
As I said in another comment, I doubt everyone here (which is not bad, it’s just not assuming by default that everything is well done everywhere) and will wait for the FAA report.