How can I synchronise a playlist with songs from iTunes AND songs from Apple Music across all my devices?

I have OS 11.1 on my MacBook and iOS 14.3. on my iPhone 12 mini. I have many playlists with songs from iTunes that I can synchronise to my mobile device. However, when I add songs from Apple Music to the playlists and enable Sync Library on my iPhone, I loose my iTunes songs. How can I synchronise playlists across all devices with songs from Apple Music and from iTunes? Many thanks, alex