Can someone explain how exactly spotify works

[user-removed]

Well I want to purcahse the monthly spotify as seems usefull. Wanting to find out some stuff first. MOST my songs atm were dled from youtube to an mp3 so they are in my pc but I can listen to them as it just syncs it to my phone and rest are ones that I found on spotify and added them to my playlist. When Im @ work and log spotify and try to listen to the songs that were only synced through my pc because they were dled as mp3 instead of through spotify is there anyway to save them permanently to spotify instead of finding each song manually and adding it through spotify.

[licenced]

Using the Spotify desktop app, put your local MP3s into a playlist. Then, with your phone and your desktop both running Spotify and on the same network, select the Devices option in the desktop app - you should see your phone listed. Select your playlist containing your MP3s to sync to your phone.