tvOS is the operating system that is used on the Apple TV, as of the Apple TV 4G. It's a forked version of iOS. The first version released was stylized and marketed as tvOS 9.0, which was based on iOS 9.1.
Since the first public version of tvOS, Apple provides a complete filesystem image in the form of an OTA update. This allows people to reconstruct the entire system partition along with all binaries, in order to perform static analysis on them.
Additionally, Apple has not actually fixed any bugs from Pangu's iOS 9.0.x jailbreak in iOS 9.1 aside from the kernel exploit (CVE-2015-6974). Because of this, Pangu9 could be modified (notably, exploiting a different kernel vulnerability) to jailbreak tvOS 9.0.
An overview analysis of tvOS OTA updates can be found in http://newosxbook.com/articles/OTA3.html.