Unable to sync Apple Music library from computer to new iPhone

Correct, you’ll need to Download the Apple Devices App from the Microsoft app store to sync the PC with the iPhone.

https://apps.microsoft.com/detail/9np83lwlpz9k?hl=en-US&gl=US

click here ➡️ Sync your Windows device and iPhone, iPad, or iPod - Apple Support

Synchronization has always been one way. PC to device. Never the other way. You cannot extract your music from the iPad with iTunes or the new Devices, this has never been possible, and has not been made more complicated, it's the way it always has worked.

Are you saying your library is not in the Apple Music App?

If you still have iTunes, then uninstall the Music App, and go back to your library there.

If your library is not visible in iTunes either to or you don't want to go back to iTunes, then simply hold down thew shift key when starting the music Apple and find the iTunes Library.itl file.

Alternatively see the following user tip.

click here ➜ Empty/corrupt iTunes library after upgrade... - Apple Community

Why not?

What happens when you try exactly?

Do you get an error? If so, what does it say?

Is your computer a Mac or a PC?

click here ➜ Sync your iPhone, iPad, or iPod using your computer - Apple Support