Are these bands mainstream or are they just known to be really good?

They're all mainstream. Metallica is by far the best out of the bunch because they are pioneers of Thrash metal. They were underground until the early 90's when they released the black album. BFMV are a bunch of *******, Korn is TOO repetitive, SOAD was really good. And nirvana is okay. Everything else is just damned awful.