How do I transfer music to an SD card on iPhone 12?

If you are talking about songs in the iPhone's Music library, I do not believe that iOS provides a way to transfer those to a SD card. It doesn't matter whether the songs have DRM or not. The Music app does not provide a way for you to export song files; the Files app does not provide a way for you to look "inside" the Music database; and this is by design. The iPhone follows a variation on the original iPod design where the master music library is on a computer, the mobile device contains a selection of music from a library on a single master computer, and copying only takes place in the computer-to-mobile-device direction.

This segment of the iPhone User Guide explains how to transfer files to an SD card from iPhone.

Transfer files from iPhone to a storage device, a server, or the cloud - Apple Support

Wherher music files are transferable is a separate question. DRM protection may be involved, for example. The music may well be copyrighted.

If you have a Mac, you can drag-and-drop songs out of the Music window to make copies of songs to places such as the Desktop, drives, folders, and SD cards.

You won't be able to use offline listening copies of Apple Music subscription songs on third-party devices. Those songs have DRM. You won't be able to use very early iTunes Store purchases on third-party devices, since those songs have DRM, too.

Songs purchased from the iTunes Store these days are in 256 Kbps non-DRMed format. Songs that you bought on CD and imported to your computer might be in various formats (AAC, MP3, Apple Lossless, etc.). While importing a CD using Apple Lossless does not add DRM, Apple Lossless is a proprietary format, and you would most likely need to convert from it to some other format if you needed a copy of a song that would work on a third-party player.

Note that the Finder likes to write various "invisible" "dot" files to non-Mac volumes (like USB flash drives and SD-family memory cards), and these may need to be cleaned off to avoid confusing a third-party player.