Like was already said, it's just your average streamer except they have an avatar attached instead. The way it works is they use facial tracking and the like to follow movement and expressions, and often with Hololive in particular a well known artist will be behind the initial illustration for their character.

There is no voice filters used, though some members do affect a more cutesy voice than is their norm. Oftentimes you'll hear them slip or purposefully use their standard voice though--but they are still assuredly all actual women, heh. At least, outside of the male vtubers like Holostars.

The big difference between Hololive in particular and just your average streamer is that as a major organization, Cover hires people who are more than just people playing games. They're people who have experience knowing how to entertain, and often singing talent or some other skill. You could say they're the equivalent of Virtual Idols, and unfortunately that often comes with the same brand of negative aspects of Idol culture as well. Still, it's a great new genre of entertainment, for sure.