I've always wondered if these people do this, and it's very possible. However, the TTS software can't be found online, and that's because it's private. Perhaps you could find the talent to make you such good TTS software online (by paying a service on fiverr or some decent freelancer website), and make one of these channels yourself. You could also find a similar one, and adjust its voice with some tuning software as necessary. The possibilities are endless. This niche has been around for a while, even back in the early days of YT, so there's no doubt that it's gotten better.