usually virtualbox uses its own VT-x thing, but qemu is in there as a fallback. also many devices are implement with code from qemu.
is basically just a gui for ffmpeg
also basically just a gui for ffmpeg
uses ffmpeg for playing video
uses ffmpeg as a library for capturing, encoding, and streaming
uses ffmpeg for merging audio and video into one file
uses youtube-dl to allow you to paste a url into the player and just play it
- davinci resolve
if i missed something significant let me know.