Adding an MP3 to my iPhone

With an iPhone (for reasons that are beyond our ken) when you attempt to either sync or manually add an item, iTunes will check every track on the device and ensure that there is a corresponding local copy in your library. If not that item is removed from the device. This makes adding content from a different library than the one originally used to load the device fraught with danger.

See Recover your iTunes library from your iPod or iOS device - Apple Community. The easiest method may be a temporary subscription to the Apple Music service, merging the music on the device into an iCloud Music Library which you then download to a computer and backup for security.

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Add to to where on your phone?

To add it to the Apple Music app, yes, you need to sync it using iTunes.

iTunes, will remove anything on the device that it does not have in its own library. iTunes works under the premise that the library on the computer is the master, so it replicates that onto the device deleting anything it does not have in the process. So, yes, syncing this lone MP3, would delete all the music from the iPhone that was synced previously from a computer. It will not delete purchases from the iTunes Store, or tracks from the Apple Music subscription service however.

You can if you don't need to store it in the iPhone's music App, use iTunes file sharing to drop it into another app.

click here ➜ Use iTunes to share files between your computer and your iPhone or iPad - Apple Support

Alternatively upload it to a cloud service like iCloud, Dropbox, OneDrive, Google Drive and access it in the relevant app on the iPhone.

With that said, you should always keep a copy of your music on the computer for future use. Should something terrible happen to the iPhone you music could be lost forever if there is no backup. And, no, synced tracks are not backed up to iCloud.

RobM77 wrote:

I need to put the MP3 into my music library on my phone; this is accessed by an app I use for cello practise. The Apple Music app has a section for your own stuff in your library - that's where it needs to go.

In that case, only iTunes can do that as explained above.

I understand what you're saying, but iTunes hasn't worked on my PC for several years now. If I sync it just wipes my phone and doesn't care what I've got ticked and what I haven't.

Yes, as explained, that's how iTunes works. Anything not on the computer you are syncing from is deleted on the device.

I did recently add an MP3, but the iTunes interface is so confusing that I can't remember how I managed it - I'm sure there was a simple "add to device" option.

There is no add to device. Never has been. iTunes works by syncing the existing library onto the device every time.

You might have just dragged the song from the song list in iTunes to the iPhone on the side bar. I believe if Manually manage option is selected in iTunes Preferences, this can be done, but have not done it in a long time.