Specifically, the BFB 17-28 source files are the native used by the animators at jacknjellify. These files represent the raw DNA of the show. To a fan, they are a blueprint of a masterpiece.

Opening these in Adobe Animate CC (or an older CS6 version) would reveal the timeline. Typically, a 6-minute episode spans thousands of frames. The source files would show you precisely how they composited the voice acting (waveforms on the timeline) to the mouth movements (symbology changes).

If you are searching for these files right now, ignore the sketchy "free download" links on pop-up ridden forums. Most of those are malware.

: The creators sometimes leave real-world photos (like dogs or internet memes) in the library to use as drawing guides. Non-Canon Notes