How do I restore songs missing from iPhone 16 plus Music playlists (iOS 18.6.2) but present on macOS 15.6.1

As the title says, several of my Music playlists on my iPhone 16 plus (iOS 18.6.2) have mysteriously been hollowed out of their music. I.e., the songs that were in them are not there. The songs no longer appear in the library either. I don't know exactly when this occurred because I haven't listened to some in a while. When I go on my MacBook (Sequoia 15.6.1), the missing songs are still intact in the playlists.

Main question: How can I get the iPhone playlists and songs to return to the MacBook original state?

Second question: If I delete a playlist on my iPhone, is there a way to restore it short of going to a backup from an earlier time? Thanks in advance.

[Re-Titled by Moderator]

Original Title: Songs mysteriously disappeared from iPhone 16 plus Music playlists (iOS 18.6.2) but still in Mac OS 15.6.1: How to restore to iPhone?