Why is it possible for my 2nd monitor to mirror my main monitors display, but when I separate them, the 2nd monitor cannot match that resolution anymore?

Why is it possible for my 2nd monitor to mirror my main monitors display and be the same resolution, but when I separate them, the 2nd monitor cannot match that resolution anymore?

the first picture is of the screens mirrored. The 2nd picture is of the screens not mirrored. The same resolution is still on the screen on the left, however this is now the maximum resolution on the screen on the right (clearly much bigger icons, etc...). If the monitor on the right is clearly capable of mirroring a screen of that resolution, why can’t I keep it that way when it’s not mirroring? I don’t need it to be as clear as the other one, I just want everything to be that same SMALLER size, so that I can fit more stuff on the screen. If there is a way to do this PLEASE tell me how...

If you know for a fact that there is NOT a way to do this, that would be helpful as well so that I don’t waste anymore time.

(I’ve already tried the 1080p option, the icons might be a little smaller but barely noticeable. I’ve also done the option+scaled to get “all available resolutions”. There’s nothing higher than the resolution in the photo.)

Thanks for any and all help!