Is it "fair use" to use figures from a publication in monetized educational material?
That is very problematic. Even if the author would allow it, the copyright is usually owned by the publisher and they are usually not interested in sharing. And even if they would allow it in principle, they are certainly not interested in answering letters from people that want to make Youtube videos.

Things are differently if the material is published under a less restrictive license, for example Creative Commons. Then you just have to follow the rules of the license. But even if the paper is published under Creative Commons, that does not necessarily hold for the pictures and secondly it only holds for very few publications.

That said, there are rules such as "Fair Use", but you have to consult a lawyer to get to know if these apply in your case. Have a look at Can I use data and images found online in my project report, under the fair use clause in copyright law?