How did I not know about this before?
Just over ten years ago the Japanese company Crypton Future Media released computer software that can be used to generate a female singing voice, originally in Japanese and later in English and Chinese as well. A fairly straightforward user interface lets you enter words and music for the virtual singer (derived from digitally manipulated samples of voice actress Saki Fujita) to perform.
The product makes use of Yamaha's Vocaloid technology, which had already been used to create other artificial singers, but Crypton cleverly gave their version a name (Hatsune Miku, which I'm told means something like "first voice from the future" in Japanese) and a cartoon drawing of her created by manga artist Kei Garō. Hatsune Miku was originally marketed to professional musicians and music producers, but she soon attracted a lot of amateur composers and large number of fans around the world. Free animation software called MikuMikuDance (often shortened to just MMD) and later MikuMikuMoving (MMM) made it possible to create music videos that were no doubt a major factor in her rapidly growing popularity. (For more history, see the Wikipedia article.)
Here's an example, the first Hatsune Miku song I listened to, and a fairly catchy one:
When a recording star gets popular, fans naturally want to attend a live concert. You might think this would be a problem for a computer-generated singer, but take a look at this performance in Tokyo, with Hatsune Miku on stage accompanied by human musicians. She's appeared around the world before huge cheering audiences.
By the way, lots of sources (including the Wikipedia article about her) erroneously call her appearance above a "projection hologram" out of a misconception about what "hologram" means. Holograms can be used to create three-dimensional images, but in general 3D images aren't holograms, and what you see above appears to be a straightforward 2K or 4K video image projected onto a thin transparent screen made of a fabric such as that used for sheer curtains. The same basic technique can be used for home Halloween decorations. See, for example, this family of products. (I'm not sponsored by them or endorsing them; I just found the link by Googling in order to illustrate the simple but effective technology involved.)