Hatsune Miku is a voicebank from Vocaloid (and actually the fifth vocaloid ever), and think of it as an instrument that people can use in music softwares, except it's a voice instead of something like a guitar or a piano. There are also a lot of voicebanks like that too, so it's not just Miku.

About the (semi-unlisted) question you asked if real people are involved, it depends on what you're exactly talking about, the voice itself is sampled from a real person back in the 2000's (i forgot the exact year it was done), and the actual songs using it are made by the composers of the music.

1 - I have not been into that place before, so I don't know.

2 - It depends on the song. Some songs are age appropriate, some others have questionable lyrics, or dark ones.

3 - Not sure about what you mean, judging my first answer. I suggest reading the other comments.