For example, in .tex files, the expression
$\sum$ is replaced with
∑. I would like to yank
∑, but when I try I get one of the characters that compose
$\sum$, even when I set
concealcursor. What is the simplest way to do this?
Concealing characters is just a visual representation of the physical text. Vim commands however work with the actual text, so that yanking actually yanks what is in the buffer and not what is displayed.
If you want to capture what is displayed, there are a couple of ways around it. The easiest solution is, if you are using Vim in the terminal and let the terminal handle mouse selection by pressing shift when visually selecting with the mouse. Since the terminal handles the mouse selection, you can paste it using middle mouse button wherever you need it (and the terminal is not aware of what the actual text is, it will copy verbatim whatever is on the screen, like numbering, list characters etc.).
Another solution is to make use of the various
screen...() functions provided by Vim. E.g. Start reading at :h screenattr() and make use of whatever function you need. Those functions have initially been created for Vims test suite to make assert what is displayed on the screen. But of course you can make use of it.
See also this answer on how to copy what is displayed in the current viewport. For your usecase, you just need to get rid of the loop and use
winline(). Note, you might need to adjust the line number for the tabline.
Update: Note, there also exists the synconcealed() function, that returns whether the item at the given position is concealed.