Here are the jury instructions.
These describe how to evaluate witness testimony, burdens of proof, and the four-factor fair use test, among other things.
The jury had to answer yes or no to the following question:
Has Google shown by a preponderance of the evidence that its use in Android of the declaring lines of code and their structure, sequence, and organization from Java 2 Standard Edition 1.4 and Java 2 Standard Edition Version 5.0 constitutes a "fair use" under the Copyright Act?
They answered yes. Juries do not explain their reasoning (different jurors might even have different reasonings), but the assumption is that they followed the jury instructions to arrive at this conclusion.
To be clear, the issue wasn't reimplementation of 37 Java APIs, but a more limited taking, including "the declaring code and the structure, sequence, and organization".