iTunes store importing music to iphone, exclamation mark problem

Hi, I use iTunes store when I want to import music (not bought from apple) from the computer to my iphone.

I just copy the songs from the desktop and paste them into iTunes store (not in the library, but in the device's music)

Today, I decided to organize my songs by renaming some of them and delete the duplicates. I don't know why I often find many duplicates of the songs on the device, appearing out of nowhere. So I did that.

Then, I realized that some of the songs that appeared on iTunes device's music were not on my actual iphone. When I tried to play them, an exclamation mark appeared. It did nothing.

This had happened before and I just re-transferred the songs into iTunes.

How do I locate all of the songs with exclamation mark?

How do I solve the exclamation mark problem?

Is there any way to remove the exclamation mark and make the songs playable again?

Thank you!