It’s just streamers with an added layer of anonymity and roleplaying (although the Western indie side doesn’t really stick to those as religiously as the Japanese corporate one, and even there the roleplaying aspect is pretty light most of the time). And streamers, conceptually, I wouldn’t say are some radical new idea, just a modernization of radio jockeys and talkshows/variety TV shows of old (although with a much wider spectrum of quality, those older formats needing an upfront investment and actual running costs kind of ensures a base degree of quality and competency, whereas streaming is a lot more manageable for a single person to independently go in, although there’s still some tech stuff and equipment that has to be figured out).

There’s going to be a lot of variation here though - there’s pro-gamer streamers who focus on playing a very specific game pretty much all the time (and those are kind of incomprehensible to me too), they could be doing challenge runs of various different games, singers/musicians who might do karaoke/cover streams, there’s reaction content of course, etc… Most vtubers are going to be more so in something akin to a variety talk show category, kind of mixing playing games and just chatting about inane bullshit. They might have some general theme - Clio was a history teacher I think, and generally plays strategy games and the like - or they might be pretty random. That’s actually an interesting aspect of vtubers specifically, you seem to get a lot more of “person with a somewhat interesting past career” people than you generally see among live streamers, or at least the successful ones (probably because requiring people who are comfortable on camera already filters the available population down quite a lot).

Although tbf, I mainly experience streamers through clipped content. The only actual streams I occasionally watch are already-recorded VODs mainly of the “person plays some game I like for the 1st time” subgenre, which could just as well have been non-live Let’s Plays, aside from the very occasionally funny chat interactions.