Transfer songs from my Windows 10 laptop to iPhone 12 Pro

See Turn on Sync Library with Apple Music - Apple Support. When you have a subscription to Apple Music you would enable Sync Library AKA iCloud Music Library in both your library and your device. That way new content you add to either will automatically sync to the other. If you've already done this, but new additions to your library don't show up on your device see Identify cloud status icons in your music library on your Mac or PC - Apple Support.

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See Sync your iPhone, iPad, or iPod using your computer - Apple Support.

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