It’s impossible to create smart Apple Music playlists directly on the iPhone. Or rather, it was impossible. Previously, you had to fire up iTunes on your Mac or PC, create a smart playlist there, and then let it sync to your iPhone over iCloud.
Even in iOS 13, this is still the case. But now there’s another way. A new iOS app called Miximum can create smart playlists, and even sync them to the regular Apple Music app. It is, as they say, a game-changer.