Errr, yes?
A video streaming service is literally a host for video content + UI's for accessing it.
Whether YouTube internally stores those video's as single files, as multiple versions of a file, as chunked pieces, or whatever... who cares? They'll obviously optimise their backend storage for their particular needs.
Whether you start viewing in the middle of a video, or from the start... are you're trying to say it makes a difference?
Because VLC can seek around a video too.
With YouTube-dl, I download the video, and watch it. At some point, it gets deleted.
With Firefox, I download the video and watch it. At some point, it gets deleted.
Are you saying it makes a difference whether or not it gets assembled into "a file" on the receiving side in order to watch it in an appropriate player?