If you think about it logically, then all music is cover.

There are four main cadences which then can be played in any key. All music is a series of permutations of this theme.

Regarding lyrics. All songs stem from the idea of putting words to music.

Yeah, it's all cover. I bet the metaphor I am shoving into the song I am presently working on has been used many times. I reckon I may even be inadvertently plagiarising as although I reckon it's mine, I probably heard if elsewhere over the years.

(I once googled a line I came up with and found it was from a '70s disco song. I was positively strutting all day with a smirk on my face. )