- If I draw an animation and post it to YouTube, and one of the characters happens to be Mickey Mouse, that will be legal. But I still can't name my channel "Mickey Mouse Official" or put the character's face in my channel profile, since that's source-identifying material.
- If I just flat-out reupload Steamboat Willie to YouTube, with the (possibly incorrect) title "Walt Disney's FIRST EVER CARTOON", that also will be legal - because the title is purely nominative and does not imply that I'm licensed by Disney.
- If I release STL or STEP files on Thingiverse for printing Mickey Mouse christmas ornaments, that will be legal - but I have to make sure that nobody thinks this is actually made by Disney.
- Mass-produced merchandise sold in stores will be very difficult to sell legally, since generally speaking the whole object is considered source-identifying when you put it on a store shelf. About the only thing you could do is sell figurine blind-bags with no indication that there's public-domain Disney stuff in there.
That last one is probably why Disney isn't trying to, say, push Mexican life+100 terms[1] on everyone. Mickey Mouse is more valuable as a branding and merchandising tool than as a creative work.
Copyright law itself also has a preemption clause[2] which prohibits making copyright-shaped claims under other laws. This is usually mentioned in the context of state right-of-publicity laws[3], but the text of the clause would also apply to trying to "trademark a copyright" to keep the mouse in his cage.
[0] This is part of "trademark fair use", which is an entirely different concept to the copyright fair use one.
[1] Oh, yeah, I forgot - in all those YouTube examples you need to convince YouTube to block your upload in Mexico, which they are unwilling to do. The stated reason is that pirates could be harder to catch if they geoblocked their uploads. However, this already causes problems for, say, people reviewing anime - which is actually illegal in Japan! So I suspect that YouTube might have to change their policies on this at some point as more large publishers' work hits the public domain in certain countries but not others.
[2] 18 USC 301
[3] https://www.sheppardmullin.com/media/article/753_CL%2025-4%2...