Effects that switch the power and toughness of a creature are always applied last. Why? It’s just easier to calculate.

For example: Lets start with a 1/4 Calcite Snapper and switch it power and toughness via its Landfall triggered ability so it is now a 4/1. If at the beginning of the combat step it get +2/+0 from Battle Rattle Shaman‘s triggered ability, the Snapper will end up as a 4/3. This is because when calculating the power and toughness of this Snapper you start with its ‘base’ 1/4 characteristics, add the +2/+0 from the Battle Rattle Shaman ability and then finally apply the switch effect from the Snapper’s Landfall ability. It does not matter that the Landfall ability resolved first and that the switch effect was generated first. If the timestamp of these effects mattered, there would be too much of a margin for error where players could forget which effect applied first.