Q: Delete a song from iPhone and not watch?

Hi

No - if a song is on your synced playlist and you delete it from your iPhone, it will also be removed from your watch.

This will happen automatically the next time you place your watch on the charger (with power connected) whilst the watch is connected to the iPhone via Bluetooth (ie the same circumstances under which you initially synced the playlist to the watch).

More information:

Listen to music on your Apple Watch - Apple Support