How are these guys building huge youtube health channels?

Both channels are outsourcing their video creation. They paid inexpensive/affordable multiple talents from 3rd countries, who can work with Camtasia, Photoshop, Text-to-Voice software and have access to Pixabay, Unsplash, Videoblocks, or even an affordable yearly Envato Element account. The cost can start from as low as $2 or $3 per minute video, depending on the contract size.

You can do it as well with MS-Power Point and recorded TTS file using Audacity. But if you want to beat the time, you won't do it by yourself. You'll need more people to help you create more videos to save times.

There're also some video creator software that can make such video style automatically like videospinblaster and its alternatives.

But looking into the channels you said here, I'm pretty sure they were hiring outsourcer.

Thanks.