> MIT/Expat does required you to retain a copyright notice (in the form of a copy of the license, not in the form of file headers)

OK, but I was talking about the file headers. :)

> In addition, creating a derivative work does in fact give you copyright on the new work

Sure, but the new work is the portions that you've changed, not the portions that you've copied, right?

"The derivative work cannot be an uncreative variation on the pre-existing work or it would simply be a copy of the pre-existing work . . . " from here: https://bit.ly/3c21Yul

>OK, but I was talking about the file headers. :)

Gotcha, then you are correct. The MIT/Expat only requires: "The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software". As long as they are in compliance on that regard then they are in the clear.

>Sure, but the new work is the portions that you've changed, not the portions that you've copied, right?

No, the new work is the piece of software work as a whole, not the individual files. "Work" in this context is a legal term that includes all of the source code and nonliteral elements of the software, aka the Structure, Sequence, and Organization https://en.wikipedia.org/wiki/Structure,_sequence_and_organi...

R.e. your second statement, I can only presume that you're right, but I'm confused about how this works with copyright license agreements, or the cases where projects have had to go and get copyright releases from authors of individual lines of code to make a license change. If the copyright is on the entire work, how can a contributor of just one line of code own the copyright? Anyway. You seem more versed in this than I am, so I bow to your expertise.

In that case, there would be multiple authors of a single copyrighted work. I'm only familiar/knowledgeable with GNU copyright assignment, however, not copyrights in general. https://www.gnu.org/licenses/why-assign.en.html