Well, costs go before the semicolon, but magic jammer has no such symbol because it doesn't have any cost.
Think of it this way: costs happen before the chain resolves, effects happen during.
Almost true. Targeting is also considered part of the effect even though it happens before resolution. That's why cards like the Kozmo ships say specifically that they "Cannot be targeted by card effects.", rather than "Cannot be targeted to activate a card or effect." which is the wording that the Atlanteans get for triggering off being used to pay a cost. This is also why you get to choose targets for Normal Spells, such as Monster Reborn, that are milled by Diamond Dude even though you don't pay the costs for those Normal Spells, such as discarding for Destiny Draw.