ffmpeg allows many lossless operations. Lossless meaning without re-compression. See lossless-cut - https://mifi.github.io/lossless-cut/ and https://forum.videohelp.com/threads/398507-Top10-Commands-for-Lossless-Video-manipulation-using-ffmpeg-%28Guide%29
I need NLE that allows me to replace the first few seconds of a video with another one, keeping the audio intact. ffmpeg cannot do this - from what I know.
Most simple way can be split the video (can use avidemux or such), make new small first part and join video again.
Compressor and VLC both have "passthrough" options, I use Final Cut or even Quicktime 7 to cut .H264 files w/out recompression.
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