How parlay payouts are calculated
A parlay's true odds are the product of each leg's decimal odds. Books pay out at those true odds minus a multiplied vig - which is why standard parlays have some of the highest house edges on the board.
When parlays can be +EV
Correlated parlays (e.g. team total + game total + game spread on the same matchup) can be +EV when the book prices the legs as if they were independent. Most books restrict the most obvious correlations, but the principle is real.