I can answer based on how it works for me. I have a hybrid nvidia setup. Laptop + occasionally additional monitor on Plasma (testing branch).
As far I remember, during a boot only main, laptop monitor shows GRUB, rest is just blank.
Nope. If I use system lock, it shows on all monitors. However, if I enter input (password), it shows it only on primary monitor, while other is still giving a lock screen till the computer is unlocked.
Login screen shows only on the main screen for me. The rest is black.
I don’t understand what do you mean. Or maybe you mean the shutdown options screen? I hated it, so I turned it off. If I choose shutdown, it shuts down immediately, without asking anything, just as I like it. So basically: shutdown button, shutdown icon on the panel, shutdown in start menu. Hit and computer is off.
As said earlier, my second monitor is plugged in once in a while, and Plasma just remembers the settings. The first time you plug in, you get some defaults and must configure it separately for that screen (panels, dock, widgets, wallpaper, screen mode, etc.). If you use latte, you may want to decide to duplicate some panels to the other screens, while keeping others out of it (dock), but it’s up to you to configure latte properly. I don’t use system panels, so I’m not sure how it behaves in multimonitor setup.
What do you mean by saying “enable”? If the monitor is plugged in, the output is already there, even if the screen is turned off. This may cause some windows to show up in that inactive screen. This is because Plasma tries to remember where the window was last time and put it there. So if monitors are plugged in all the time, they should be turned on. If you plug them in and out, there is no problem, Plasma will remember the setup and if some window was opened in this setup in other monitor. Additionally, Plasma follows the active screen, meaning, where your mouse or active window is, so some visible outputs are showing on that screen, like volume +/-, switching to different keyboards, etc.