How YouTube stores video, and why it matters
A single YouTube video is not one file. Understanding how it is actually stored explains most of what a download tool does.
Separate streams, joined on playback
For most uploads, YouTube keeps the picture and the sound as two independent streams and combines them in the player as you watch. This lets quality change mid-playback without re-fetching the audio.
It also means we cannot simply hand you one ready-made file. The two streams have to be joined back together first, which is the step the progress bar is showing.
The codecs you will see
Video is typically H.264, VP9 or AV1. H.264 is the oldest and the most universally supported; VP9 and AV1 are more efficient but less widely playable on older hardware.
Audio is typically AAC or Opus. Opus is the better codec at low bitrates; AAC has broader support. An MP4 download pairs H.264 with AAC, which is the combination that plays everywhere.
Why "progressive" formats mostly disappeared
YouTube once served combined files containing both streams, which is why older tools could offer an instant download with no processing step.
Those combined formats are now limited to low resolutions or gone entirely. Any download above that is produced by merging, and a tool that claims otherwise is either giving you low quality or giving you a file with no sound.