Hatsune Miku is a digital musical instrument, a piece of software which allows people to generate singing in a robotic Japanese voice by manually tuning each note. This is different from "AI" voice song generation because it actually requires work and skill and predates it by at least a decade. The blue haired character is the mascot of this piece of software, and the company behind it uses that mascot in various tie-in products like video games. There is no specific real human behind the Miku character.
Vtubers are like twitch streamers, but they're behind a fancy mask called a "model" which is animated by face and/or body tracking. Each one corresponds to a specific real human. The big/popular ones are part of agencies, generally out of Japan with corresponding Japanese-style workplace abuse. There are also people who do vtubing on their own, and generally have much less detailed and less animated models, all the way down to just bouncing a png of a face when they talk.
Yes, live performances by both are just on a screen, but with the other things I'm told people like about live events, like a big soundsystem and a crowd of fellow fans.
Yes, pretty much. Vtubers stream like other streamers but show their models instead of their own faces, and lots of them do karaoke streams or even original songs, which leads into the concerts. Graduation is retirement either from the company that owns their model or from streaming entirely. In the former case, they may pop back up later with a different model either streaming independently or with a different agency.
For an example of Miku's licensing, probably the most famous Miku song is "World is Mine" which was written by a composer named ryo and performed by his band Supercell. I'm not sure exactly what deal was struck between their label, Sony Music, and Crypton Future Media, the makers of the Vocaloid software Miku is the mascot and most popular voice of to make the concerts. Anyone is allowed to use the Miku character for noncommercial purposes with attribution, her image is CC BY-NC.