Apple has support articles on where to find your iTunes music, for example:
Back up and restore your iTunes library on your PC - Apple Support
The actual songs are in subfolders, sorted by artist. How big is your library? It might make sense to copy it to a microSD card – or even to consider a streaming service like Spotify instead.
Apple uses the AAC file format, which can be played on the FP3. The phone does not come with a default music player app, though. But you can easily find one in the Play Store or the app store that you prefer. There was a discussion about this just a couple of days ago:
If you would like to use Bluetooth headphones, the FP3 has a caveat: it does not support the high-quality codec aptX (HD), see
Fairphone 3 and aptX? - Discuss / Fairphone 3 - Fairphone Community Forum
Best wishes,
Thomas