This app was mentioned in 4 comments, with an average of 1.25 upvotes
Have you tried my ringtone maker app? Just let me know if you run into any issues, I've got an S6 on my desk and would love to reproduce anything you are seeing.
https://play.google.com/store/apps/details?id=ringtone.maker
Without a PC, you cannot obtain access to files downloaded via Play Store outside of their UI. They do not have an option to create a file as a ringtone and they do not share their API so 3rd party devs (like us) can access the file through another application to create a ringtone.
So, LEGALLY obtaining "Happy" without a computer, you should use a different song downloader - Amazon Music is an option, 7Digital is another. There are surely others that do this - but those are the ones we recommend to our customers.
Once you have the file downloaded, not cached (in Amazon Music's case) then use your ringtone cutter of choice. There are many ringtone apps, ours is: https://play.google.com/store/apps/details?id=ringtone.maker&hl=en
We agree that Google could make this easier for elderly & devs if they would open their API to 3rd party devs. Hope that helps!
Get a Ringtones Maker. So easy. Edit your own files and assign to contacts all in the app. I use the one by JT Audio.
https://play.google.com/store/apps/details?id=ringtone.maker