How to save a file using GNU Nano? I use Termux and GNU nano to edit program text but I don't know how to save it from Nano. Normally in Nano I press shift + O but with Android (I use Samsung S8) the ctrl + O instead prints the char O. 

How should I save and exit Nano?
You can exit by using (ctrl + o) not (shift + o)

Tip. In termux use vim instead. 
not because of "text editor wars" but it is painful to send ctrl and shift keys even with modified keyboards similar to hacker's keyboard