Probably through some combination of:

1. Publish higher-quality content on the verified channel, and lower-quality closely related content on unverified channels. Suggestions on the unverified channels are more likely to include content from the verified channels than content from elsewhere, as that content is more closely related.

2. Get multiple channels verified! If it can be done once, it can be done multiple times.

this is what fascinates me - how do people actually monetise this stuff

I was amazed to discover the Nigerian scam was a means to get money out the Western banking system - find a fool who will transfer 900,000 of a million bucks to a number somewhere and you get your stolen credit card money out the country.

(see also gangs who walk down high street taking out 1000 in cash at each ATM)

But this one - YouTube is paying these people. what about fraud prevention are we missing here?

Personally, I think Youtubes recommendation algorithm is to blame. It's probably to easy to fool; have a nice clickbaity title, some clickbaity thumbnails, and you're much more likely to get hits. YT should eliminate those factors, as it only lowers the bar, but they don't.