FFmpeg

FFmpeg has existed since 2000 and is one of the most foundational pieces of open-source software in the multimedia world. VLC, Handbrake, OBS Studio, Plex, Kodi, YouTube's encoding pipeline, and most online video tools either embed FFmpeg directly or wrap it in a higher-level interface. When you see a website that converts videos, extracts audio, or downloads from a streaming protocol, the heavy lifting almost always happens inside an FFmpeg process running on the server.

DropZap uses FFmpeg server-side for two main jobs: merging Reddit's separate video and audio DASH streams into a single MP4, and transcoding videos to MP3 audio for the audio-extraction feature. Both happen on the server in a few seconds, so users don't have to install FFmpeg locally or learn its (admittedly intimidating) command-line syntax.

If you're ever curious about manually doing what an online tool does, FFmpeg can almost certainly do it: ffmpeg -i input.mp4 output.mp3 extracts audio, ffmpeg -i video.mp4 -i audio.mp4 -c copy merged.mp4 merges separate streams, and ffmpeg -i playlist.m3u8 -c copy out.mp4 downloads an HLS stream as MP4. The catch is that most platforms' video URLs aren't directly accessible — you need to first parse the manifest, which is the part downloaders automate.