I’m adding ringtones via Termux because you can script the process! Instructions assume a ~ (Termux home) start point.
Download the file from wherever you’re keeping it. You can use curl or some other such thing. There are some Creative Commons files you can get off YouTube with youtube-dl (although YouTube frowns on downloading their videos at all).
youtube-dl --extract-audio --audio-format mp3 <link to YT video> You then need to move this to the Ringtones folder (or Notifications or Alarms depending on your usecase).