Open Spotify
Search for the song
Select the song from the results
Tap or click the three dots next to the song
Choose Add to Playlist
Select an existing playlist or create a new one
To save it to your library, tap or click the heart icon if available
To add it to your liked songs, select Like or Save to Your Library
To add your own music file, enable local files in Spotify settings
Add the music file to Spotify’s local files folder
Refresh Spotify to import the file
Add the imported song to a playlist if needed