My computer died, how do I transfer all my ITunes to my iPhone 12

If all of your iTunes music was purchased from the iTunes store, or if you subscribed to iTunes Match, you can open the Music app and download it, or the iTunes app and download it. Music from other sources (ripped CDs, MP3s, etc) will not be available unless you have iTunes Match.

The iTunes Store app is on your phone somewhere. Use the Search to find it (pink icon with a white star). If you deleted it you can download it again from the Search or the App Store. Once you find it tap on the 3 dots at the bottom, then Purchased.

you can also use the Music app, as deggie suggested.

You need to use the Music app.