By the time 1.4 ends, the planner has produced one PlannedStmt. Inside it is an execution tree built from Plan nodes, frozen into a form you can follow step by step, something like "go into the primary key index on users, fetch the one matching row, then output that whole row." But that is still only a blueprint. Reading actual pages off disk, picking out the rows that match the condition, handing results back to the caller: none of that has happened yet. The stage that takes that blueprint and produces actual rows is the executor.
The difference between the planner and the executor is the difference between deciding and doing. The planner was the stage that weighed "which index, in what order, with what join method" by cost and chose. The executor takes the chosen approach and carries it out as is. There is nothing left to choose. It just runs the nodes baked into the plan tree and pulls rows out of them.
To run it, the executor takes the Plan tree it received and turns it into a PlanState tree. The Plan tree is the static blueprint the planner made, and it does not change during execution. But to actually run, each node needs state that changes as execution proceeds: which row it is reading now, whether the hash table is fully built, what tuple it has buffered from a child. So when execution begins, a PlanState tree with the exact same shape as the Plan tree is created. The blueprint Plan tree is left untouched, and the running state lives in that PlanState tree instead.
