Losing my own music after cancelling a three-months free trial of Apple Music

How did this music get onto your new Mac originally?

If you activated the Sync library function from an older Mac that contained your music library, then your Mac made use of the Apple Music subscription. service to get the music from the old Mac through iCloud and onto the new Mac. This means it was entirely dependent on the subscription to access the music. Once it ended, it no longer has access to the music.

If your Music still exists on your old Mac, you can transfer it from there to the new Mac.

click here ➜ Move your iTunes library to a new computer - Apple Community

The iPhone very likely had your music synced or transferred directly from your old computer so the Library Sync over Apple Music did not apply and would not affect it.

There is no theft here only a misunderstanding on your part of how Apple Music works.

If your music is no longer accessible anywhere else but your iPhone, then you will need to extract it from there to get it to onto your new Mac.

click here ➜ Recover your iTunes library from your iPod or iOS device… - Apple Community