Hi pauldo,
I'm the fretless bassist, Justin, in the video above. Thank you for the kind word!
You are correct. The parts were tracked/video-recorded separately.
I used three programs to create the video.
1) Finale (music notation software) to create the arrangement and everybody's notated parts.
2) VSDC Free Video Editor (video-editing software that is free, but I paid for an upgraded version) to stitch all the videos together. Also used to take out and put in audio.
3) Audacity (free audio-editing software) to bring all the audio together then edit/mix it.
Honestly, I didn't really know anything about using audio- or video-editing software before the pandemic. It's all very new to me, but I'm having fun figuring it out.
Justin