Yes, exactly. If you for example rewrite parts of Shakespeares Romeo and Juliette you have copyright on the changes you made. So the parts you didn't change are

Yes, exactly. If you for example rewrite parts of Shakespeares Romeo and Juliette you have copyright on the changes you made. So the parts you didn't change are in the public domain while the parts that are changed you got the copyright for (assuming the changes reach the level of copyright and aren't just for example spelling errors). Same with code. If you extend a MIT library you have copyright to the changes you made but not the parts that aren't changed.