Can't play non-Apple Music songs on iTunes (PC)

I was on holidays recently and downloaded and edited some songs from the Internet onto my MacBook Air (Late 2010). It runs on macOS High Sierra.

Then I put those songs on my iPhone (iOS 14.7.1), and they're playing just fine. Now I'm trying to transfer those songs from my iPhone into my iTunes library on my PC. I synced the library, however, the songs are grayed out.

Why is that?

[Link Edited by Moderator]