I think it's important to realise that anybody can be influenced, even smart people who look at any information with healthy criticism.
Unfortunately, the human brain is not without its problems. One of those problems is that information that is repeated often will start to "feel" true, even if it is false. If you want to read about these kinds of biases, I'd recommend the book Thinking, fast and slow by Daniel Kahneman.
With platforms like YouTube, it's incredibly easy to get stuck in a filter bubble. Their recommendation algorithm is more than happy to keep recommending you conspiracy theories, because users seem to like those, and the algorithm doesn't care about the actual content.
And once you've watched 10 of those videos, the 11th will no longer sound as ridiculous as it would have if you had watched it without first being (perhaps subconsciously) influenced by those first 10 videos.
This is a real risk of algorithm-based recommendation platforms, and something that Google/Facebook are trying to address now because it has already caused real problems in society.