Sync MP3 files from iTunes on Mac to Apple Music on iPhone
duskwuff's answer is correct if you have a subscription to the Apple Music or iTunes Match services. An Apple Music subscription gives you access to the entire Apple Music catalogue from any device signed in to your account; iTunes Match only lets you download a copy of the music you've imported into iTunes on your computer.

If you don't subscribe to either service, you should be able to sync MP3 files from the Mac to the iPhone by connecting via USB. Be aware this may remove music that's already on the iPhone, but you'll see a warning first, and if you had bought that music from the iTunes Store you'll be able to redownload it.