Is time sharing an illusion?

I understand that every modern human made computer is being operated by time sharing operating systems which allow two or more users running two or more processes at once each, from two different terminals.

Take for example playing two songs at once: One from YouTube and one from MP3; they appear “mixed” but the “particles” of each one are being played alone in a tremendous velocity so to the human mind it appears “mixed” but not really is.

Is time sharing an illusion?
Is it true to say that even time sharing operating systems still handle each process alone in tremendous velocity so that the time sharing is just an illusion of “present” but is actually only the past and the future so two or more processes can’t really be handled by one single processing unit at the exact same time?