Your Moto G5 Plus features a microSD memory card slot. If your personal computer is compatible with that type of card (Most laptops are so-equipped), you should be able to save a music file
The Motorola Moto G5 Plus is an Android phone and there are plenty of free ringtone apps such as MP3 Cutter, RingDroid (open source) and more.
Here are some of the free ringtones (Motorola Moto G5 Plus-compatible) offered here at MartinsRingtones.com:
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