how do I transfer music files from ipod nano to mi iphone 12pro
The NANO comes up on my windows PC as a MAC and wants me to reformat it that will lose all my music on it. I would like to be able to put all the music onto my iphone 12 pro
Normally, one would use iTunes.
Since music is added to a nano via iTunes, that music should still be in your iTunes library. You would then use that iTunes library to copy the music onto your iPhone.
I'll take a guess that your music was on a Mac, but now you're using a PC. In that event, the usual method is to copy the music from the Mac and paste it into your PC, then use iTunes on the PC to copy the music to the iPhone. Do you still have access to the Mac, or the drive that had the music on it? Alternatively, did you make a backup of your music that you could use?
iTunes does not have a method to copy the music from a portable device to iTunes, so if you no longer have access to the original library, you will have to use a third-party programme to retrieve the music from the nano. See this post from fellow user turingtest2: Recover your iTunes library from your iPo… - Apple Community.