The Nokia 6 2018 is equipped with a microSD memory card slot. If your laptop or personal computer is compatible with that kind of card (Most notebooks are so-equipped), you should be able to transfer music files
The Nokia 6 (2018) is an Android smartphone and the Android market offers a bunch of free ringtone apps such as MP3 Ringtone Maker, Zedge and more.
Learn more about making your own free ringtones
Here are links to some of the all free ringtones (compatible with your Nokia 6 (2018)) we offer:
Free 'Unclassified' Ringtones Free Tropical Ringtones Free Ringtones-a-gogo Free Techno Ringtones Hear All Our Free Polyphonic Ringtones