As you can see here I have a song with a title and an artist, however if I were to upload it, it would still say it is untitled by unknown.
This is because MP3 and most other sound file formats have a specific place where it stores the artist info, title and other information about the song.
However if you are uploading to other places such as Bandcamp or Soundcloud, you may want to fill out the other sections. Now if you notice it says in grey text to the right of the file name, the track #, Title, Contributing artists, and album. If you have any troubles or questions feel free to leave a comment or message me.
I followed all the steps, but on Details page no entry on Contributing Artist, Year etc were accepted.
On an i Phone, i Pad, or secondary Mac, this process is different: Any songs you play on those devices are sourced directly from your i Cloud Music Library, and even if you download them locally, they can be removed from your device if your i Phone's storage space dips too low.