Subcription + track charges

If you're talking about Apple Music, that's a subscription to listen to the music for as long as the subscription is active. You also have the option of buying a track. Make sure you are downloading the Apple Music version for offline use.

Add and download music from Apple Music - Apple Support

Limnos is correct.

iTunes still exists as a way for people who choose not to pay monthly for AppleMusic.

So if you Subscribe to AppleMusic, do not open iTunes on your mobile device. This will only cause confusion.

As for "offline" use, (as Limnos' linked article indicates) make sure that you manually (or make the adjustment in settings) download all music you add to your library while using AppleMusic. While it may appear as though adding a song to your library or playlist is sufficient, you're still "streaming" the music as it resides in your library.

You’re welcome! I know switching devices and services can be a headache, especially with all the changes in the last 5-10 years!