Ok I'm going to give a shot at answering one of your questions. If I'm wrong, I hope someone will correct me. The reason why you say a song is in the key of C and not Am is because typically a song's chord progression will start with the first note of the scale, so a song in the key of C may go C-D-G, whereas a song in the key of Am would go A-C-B. Now if you were to do a solo over either of these, you could use either the A Minor scale or the C Ionian scale, because the notes are the same. did that help at all? Moreover, was I right? I'm still a beginner at this theory stuff too