I'm a self-learner and understands how major and minor chords are made up and how extended chords are build from these, like sus2, sus4 and 7th chords (if I'm correct, these are just embellishments of those major and minor chords). I also know about chords within a specific key and know that songs can be written in a specific key or written in such a way that it doesn't fit into a single key.
What beats me is seventh chords. Let's take the song, Black by Pearl Jam. The verse and intro has a simple progression, E7-A. They do sometimes when playing this live, change the A to an Asus2 chord (which is just an embellished A chord) every other time. If I'm correct here, the verse is written in the key of A because a dominant 7th is build from the dominant chord within a given key. In this example, E is the dominant chord in the key of A. That is how I understand it. So my progression would be V-I if I want to transpose this to another key.
The rest of the song is written in Em as there are other two progression in the song which consist of C-Em and D-C-Em
Now, if the progression in the verse was Emaj7-A, that would mean the song is written in the key of E because the 7th here which is added to the E is a true 7th degree of the E major scale. This would therefor be a I-IV chord progression.
My question is, is this info correct, and if not, why?. Also, this would mean that, if a progression have two different dominant 7th's, then that progression aren't written in one specific key but is basically a mix between diffirent keys