What does OVER() do? What is PARTITION BY?

OVER() defines the window over which the calculation happens.

SUM(salary) OVER ()

calculates across all rows.

SUM(salary) OVER (
    PARTITION BY department
)

calculates separately for each department.

GROUP BY:

Many rows → fewer rows

PARTITION BY:

Many rows → still many rows

It just splits the rows into groups for the window calculation. You still keep every row.