The order of the columns in GROUP BY doesn`t matter? why?

I did a little test and find out that the order of the columns in GROUP BY does not matter to ORACLE.
Why? How does oracle do GROUP BY exactly?
Could anyone show me its processing logic? maybe in pseudo-code?
Thanks in advance.

I mean,
I thought oracle would group data based on the order of the columns in the GROUP BY LIST.
For example: for select deptno, job, count(*) from emp group by deptno, job;
I thought, oracle first would group the data based on deptno (10,20,30), e.g. in deptno=30, we got 6 employees.
and then, group this 6 employees based on JOB (CLERK, SALESMAN, MANAGER).and
for select job, deptno, count(*) from emp group by job, deptno;
first would group the data based on JOB (CLERK, SALESMAN, MANAGER) e.g., for CLERK, we got 4 employees,
and then, group this 4 employees based on deptno (10,20, 30).and to me, it seems that in some circumstances, their result could be different.
So, I wonder if the order of the columns in GROUP BY Clause matter?

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