It used to be you could directly download any YouTube video in any quality you wanted, as a single .mp4 file. However, around a year ago, YouTube switched from the "single file stream", to "DASH" streaming, which streams the video and the audio to you as two separate streams, which are played in sync with each other in the YouTube player.
It's still possible to download YouTube videos as a single file, but YouTube only offers that for qualities up to 720p. So you can't download "single file stream" videos in 1080p or higher.