Hugo Kornelis digs into an execution plan:
The OVER clause, that can be added to aggregate functions to turn them into window aggregates, can come with or without a frame specification, in the form of an ORDER BY clause, plus an explicit or implied ROWS or RANGE clause. If there is no frame specification, then every row in a partition, or window, can see all other rows in the same partition or window for the purpose of the aggregation.
In plansplaining part 6, I looked at this specific form of window aggregation, and explained in detail all the steps that the execution plan takes to compute the aggregation result and add it to each of the rows, for every row in each window.
Click through for another look at the plan.