If you go into System Settings then Display and choose your display, you should see the display resolutions below. They default to icons with labels like Larger Text and More Space.Built-in display please. I don't have a silicon Mac with me. I don't remember what I did. I think it was via settings and option key under display or 3rd party app.
You can set the resolution larger than the physical resolution of the display using a scaled mode. SwitchResX or BetterDisplay may help to list those modes.
Note that the "Looks like 2560x1440" mode is probably using a framebuffer that is 5120x2880.
Are you running Windows 10 in a VM on both displays? If so you shouldn't need to set the the resolutions to match, you just need to set the display scaling factor to match. If the 5K external screen is set to a scaling factor of 200% (which would give the equivalent of a QHD display) then the MacBook's internal screen also needs to be set to 200%, this should eliminate issues with windows having to rescale or re-size when dragged across displays. This may need to be done in both the macOS system settings app as well as in the Windows settings app within the VM.If I run a Windows 10 application on a Silicon MacBook Pro 16" while connecting it to an external 5K display, I need to adjust the windows if I drag them across the screens. Having both the internal and external screens at the same resolution can avoid this problem. Is there a better way to solve this problem besides buying two 5K monitors? Doing that leads to another unsolvable problem since unlike PC, Silicon Mac cannot remember display settings and contents/settings get swapped after reboot or wake from sleep. Apple is aware of this problem but there is no fix.
There is no supported way to run Windows on ARM Mac without some VM software.Parallels or VMware Fusion... Is there another good way to run Windows software on silicon Mac?
There is no supported way to run Windows on ARM Mac without some VM software.