ffmpeg is, mainly, a bunch of libraries. libavcodec, libavformat, libswresample, etc, is almost all of ffmpeg. If a project is using those libraries, it's using ffmpeg.
The ffmpeg command line utility is "just" an interface to those libraries.
ffmpeg is a lot more than just a wrapper on libraries. It can do a lot of filtering and rewiring of audio channels, video channels and subtitles.
Handbrake doesn't do any of that. You can't even drag a bunch of audio files on to handbrake because handbrake doesn't do audio, while ffmpeg is great for encoding audio.
What I'm saying is that ffmpeg is the libraries. When you use the ffmpeg CLI to filter and rewrite audio, the ffmpeg command is just decoding it using a decoder in libavcodec, filtering using a filter from libavfilter, and encoding again using an encoder from libavcodec. The ffmpeg CLI is "just" an interface for the libraries.
The fact that Handbrake doesn't expose the same features as the ffmpeg CLI tool is frankly irrelevant.
The fact that Handbrake doesn't expose the same features as the ffmpeg CLI tool is frankly irrelevant.
It's not just relevant, it's the whole thing. They asked for an ffmpeg GUI and someone recommended a GUI that doesn't use ffmpeg and doesn't do what ffmpeg can do. ffmpeg can not only rewire channels, it can stream video, capture video from the screen, capture video from a tv tuner, overlay text etc.
Also the libraries you listed are part of the ffmpeg project. They come from ffmpeg.
to be fair, I have never seen a GUI that could do everything its complex command-line equivalent could do, that wasn't, in the end, just simpler and easier to use the command-line in the first place. so when people ask for "a GUI", I think a lot more information is needed about what it should be able to do.
Because HandBrake uses some parts of the FFmpeg libraries, but HandBrake scope is much smaller than FFmpeg, and while it uses some parts, it's definitely not FFmpeg CLI GUI.
I know they use some of the same encoding libraries.