Your G5S Plus has a MicroSD memory card reader. If your personal computer can also read that card type (Most laptops are so-equipped), you should be able to save a music file
The G5S Plus is a smartphone. Most smartphones will let you save any song found online by tapping and holding your finger on it, then selecting 'save.' Once a music file is saved to your media library, tap-and-hold it and select 'assign' from the drop-down menu.
The Motorola G5S Plus is an Android phone and there are plenty of free ringtone apps such as MP3 Cutter, RingDroid (open source) and more.
More on free ringtones
Here are some of the 100% free ringtones (Motorola G5S Plus-compatible) offered here at MartinsRingtones.com:
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