When sampling took off, the copyright situation was sorted out and the end result is that there are ways to license samples. Some samples are produced like stock footage hat could be pirchased inexpensively, which is why a lot of songs by different artists have the same samples included. Samples of specific songs have to be licensed, so a hip hop song with a riff from an older famous song had some kind of licensing or it wouldnt be played on the radio or streaming services. They might have paid one time, or paid an artist group for access to a bunch of songs, basically the same kind of thing as covers.

Samples and covers are not plagarism if they are licensed and credit their source. Both are creating someing new, but using and crediting existing works.

AI is doing the same sampling and copying, but trying to pretend that it is somehow not sampling and copying and the companies running AI don't want to credit the sources or license the content. That is why AI is plagarism.