That’s not what I asked, but I suspect some kind of language barrier, given your screenshot in the other thread.
Also not quite an answer to my question. Where do the apps open? I think GNOME, Ubuntu’s desktop environment, has a memory and will start apps on the same monitor on which they were seen last. And if they are started for the first time, they just launch on the monitor which has the mouse pointer in it.
Can you check if that works? It may not be what you initially expected, but it may be how it actually works. Plus, it’d make a lot of sense. Sadly, I only have my laptop screen and cannot actually test this myself.