Is it "fair use" to use figures from a publication in monetized educational material? Is it considered "fair use" (i.e. legal) to use figures and/or images from a journal/conference publication, in educational material (possibly for a lay audience) that is monetized or monetizable, assuming proper referencing is done?

Example: suppose someone has a Youtube channel that discusses science, and wants to cover some immunology. Can they display a figure from a Nature review during their video (say for a 2 minutes in a 30 min video)? Such channels do make money (or can, at least) from advertising.

In some sense, this is similar to science journalism I suppose. 

My thought is that the safest thing to do would be to ask the publisher (since I think that usually the author is not the legal holder anymore). But would that have to be done, say, for every video and every figure in each video (using the example above)?

I am not looking for formal legal advice, just whether an academic doing such a thing is uncommon, if people have some experience in such matters, whether one should feel confidently safe (or not) to do such a thing, etc...

Edit: I'm mainly interested in the USA and Canada.
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?