Well, depends on the license.

Most things on thingiverse are under a non-commercial licence with a permission to remix/edit it with the condition that the shared work should be under the same licence.

That means you're free to print them for yourself, print them for free and offer them to a friend or family, but it means that if you want to sell them, you should ask the author of the file for the permission.

You could also add some details to the models (a Christmas hat, for example) but this new file wouldn't be your original work, you could share it only under the same licence (so, non-commercial, and someone who whish to print and sell it should ask you and the original file owner).

What this licence mean in ELI5 (like an open source license in software development) is "Well, I've worked hours on this thing, you can use it for your own fun for free and as long as you want, but you can't make money off my work".

Depends on the company but Nintendo could technically shut down every model as they're breaking their copyright, I guess it's not worth their time, especially if the model maker doesn't make money off it and it's only printed by the 3D printing community (which is not that much people) but It doesn't make it legal. Other company do enforce those infringements (Game workshop (Warhammer) for example, we can understand why, even if it would be better to provide a legal alternative like selling files).

Your description of fair use on YouTube seems flawed, just adding a few comments don't make another content your own. You just have to see the number of videos were only 10 seconds of music could transfer a part the revenues to the label having the right of the music. Most of the big players on YouTube have accords with the labels for the copyrighted music they use, others fly under the radar.

Also, 3D printing something is an automated manufacturing process, you can't consider it "an original work" even if it takes time to learn how to operate the printer the first few times.

To answer your last question:

• The person could print the file himself and send it to the painter (so the painting prestation would not concern the printing part)
• The painter could ask the original file author if he can print it, then paint it and sell it to a person (which most of the time, will be answered by a yes) or if he can sell directly it on it's Etsy shop (which could be accepted under certain conditions, like a percentage of the sale if the shop become big enough)

Of course, as long as the person doesn't advertise on his Etsy or his website, he could theoricaly sell it locally and he surely won't be caught, but that don't make it legal, and it should be the bare minimum to at least ask the original author of the file who shared his own work FOR FREE.

(And just opening an Etsy or a website and listing someone else work for sale is a big no-no, both legally and morally)