The distinction between liturgical music and that composed merely for entertainment has been mentioned. But, even that distinction can get blurred.
In the Orthodox shul I attended back in the 70s, there was a brief period when the some of the younger adults would “creatively” incorporate into the kedusha of Shabbos musaf popular melodies heard on the radio and in theater (I don’t think they ever consulted the Rabbi beforehand).
Probably the most bizarre of the lot I can recall was a visitor who used the title track from the Broadway musical: ‘JC Superstar’.
The practice stopped not long after that.