Manually Add Music to Your iPhone: macOS Catalina and later
Beginning with macOS Catalina (10.15), music syncing is controlled via the Music app, but you must first enable manual control via Finder.
Switch to Manual Mode Using iTunes: macOS Mojave and earlier
When you sync music to your iPhone using the default method, all the songs in your iTunes library are transferred. To make better use of your iPhone’s storage capacity, only sync the songs you want to play. To add certain songs and playlists from your library to your iPhone, you must first enable manual control.
How to Add Specific Songs to Your iPhone: macOS Mojave and earlier
With iTunes now in manual syncing mode, you can choose individual songs and playlists to transfer to your phone.
iTunes Alternatives
You can add music to your iPhone without using iTunes. There are other programs that transfer music and other files between your computer and iPhone. If you want a free iTunes alternative for Windows, Mac, iOS, and Android, download Syncios. It supports copying music (as well as other files like documents, videos, apps, and images) to and from your iPhone. To add music to your iPhone with Syncios, open the Media folder, then tap Add to see another menu. There are two options to add individual music files to your iPhone and entire music folders. Another way to add music to your iPhone is to utilize music streaming via cloud storage services. For example, upload your favorite songs to Dropbox or Google Drive and use those apps on your phone to stream only those files instead of your entire music collection.
