How to determine which modules needed for licensing FFmpeg as part of an app
In general, the codecs involved in a pure muxing command are the decoders and bitstream parsers for the codecs of the streams in the input file.

Even though there's no decoding or encoding requested in a pure muxing command, ffmpeg uses the decoders to acquire the attributes of the streams in the input. These attributes are then passed on to the muxer, which will write in the header/metadata section(s) of the output file. This has to be done because the attributes of an encoded stream is stored in a form specific to that codec.