how to transfer music from a ipod shuffle to a iphone 6

If those songs were synced to the iPod shuffle from your iTunes library, sync the same songs to the iPhone from that same iTunes library.

If that's not possible... There is no way to transfer anything directly between an iPod shuffle and iPhone. It needs to go through iTunes. However, iTunes syncing to an iPod shuffle is generally in one direction only, from iTunes library to iPod. You cannot use iTunes to transfer song files from iPod to computer (except for songs purchased from the iTunes Store).

NOTE: If the songs are purchased from iTunes Store, you an just download them again on the iPhone or computer (at no cost)

Download your past purchases - Apple Support

For your other songs that did not come from the iTunes Store, there are third-party methods and utilities that can transfer from iPod to computer. If you do an Internet search on something like "ipod music transfer," you should get some links (including an article by CNET on the topic). Once the song files are on your computer's drive, add them to your iTunes library. You can then sync those songs to the iPhone, as desired.