This article explains how to manually sync specific songs to your iPhone using the Apple Music app (macOS Catalina and later) and its alternative, Syncios. It also includes separate instructions for anyone using a Mac computer with macOS Mojave (10.14) or earlier.

Starting with macOS Catalina (10.15), music syncing is controlled through the Music app, but you must first enable manual control via Finder.

Connect your iPhone to your computer using the cable.

Open a Finder window and select iPhone from the menu bar on the left. (It's under Locations.)