You have correctly identified where pantheons come from. The term for these religions is "syncretist". Each tribe, village, city-state has its god or gods that

You have correctly identified where pantheons come from. The term for these religions is "syncretist". Each tribe, village, city-state has its god or gods that join in, or are merged into others already there (some get lots of names). Then when the emperor wants to solidify his power, he replaces local kings with his own people, and identifies himself with a chief god. Akhenaten, Tut's father, tried it in Egypt, with Aten as big cheese god. It didn't work out (probably) because the priesthood turned out still too powerful. But that goal probably is a big part of why Rome went Christian.