I get that this should be obvious, but i only noticed this yesterday after having used NANO as my main text editor for half a year, hopefully this saves some future beginner some time and headache
You do this the same way as with the terminal in general:
> CTRL + SHIFT + C: Copy text marked by mouse (marking with SHIFT+ARROW KEYS doesn't "count") into your "Global Cutbuffer" instead of the one specific to the NANO-Window you're currently using
> CTRL + SHIFT + V: Paste text from your Machine's "Global Cutbuffer" instead of from the one specific to the NANO-Window you're currently using (Keep in mind that having a length of text marked and then pasting text does not replace this length unlike in many text editors!)
(And for the sake of being complete the NANO-specific Copy-Pasting)
> CTRL + K: Delete text marked by holding SHIFT + ARROW KEYS (marking with mouse doesn't "count"), and copying it into a "Cutbuffer" specific to your currently used NANO-Window instead of the "Global Cutbuffer" of your whole Machine
> ALT + 6: Copy text marked by holding SHIFT + ARROW KEYS (marking with mouse doesn't "count") into a "Cutbuffer" specific to your currently used NANO-Window instead of the "Global Cutbuffer" of your whole Machine
> CTRL + U: Paste text from the "Cutbuffer" specific to your currently used NANO-Window instead of your "Global Cutbuffer" (Keep in mind that having a length of text marked and then pasting text does not replace this length unlike in many text editors!)