Open Spotify
Search for the song, album, or playlist
Tap or click the three dots next to the item
Select Add to playlist
Choose an existing playlist or create a new one
To save a song to your library, tap or click the heart icon
To add local music files, enable Local Files in Spotify settings
Add the music files to a folder on your device
Make sure Spotify has access to that folder
Refresh Spotify to see the local songs
Drag the local songs into a playlist if needed