[_Gyan_]

libav is a part of FFmpeg, and the heart of it. But FFmpeg isn't just a skeleton. Most decoders + protocols + demuxers / muxsers + filters + scaler + resampler are native components.

The most well-known 3rd party libs provide a few (albeit commonly used) video encoders. Of course, having interfaces for these external libs is what allows media operations to be consolidated into a single process workflow. As standalone tools, x264/x265 would be more cumbersome to integrate into a pipeline.