You have been told they are the same thing. Roth is Roth. Pay tax now, withdraw free later, done.
That is true for about the first eleven years.
Then you retire, and the two accounts start behaving like completely different animals. One lets you reach your own money. The other does not. One has a clock that started decades ago. The other may have a clock that started last Tuesday.
And one of them will happily hand your heirs a much worse outcome than the other.
The strange part is that during your working years, the Roth 401(k) is usually the better account. Bigger limit, employer match, no income test.
Then the moment you stop working, the advantages flip.
Most people never notice the handover. They retire holding a Roth 401(k), assume the job is done, and leave it there because moving it seems like paperwork for no reason.
Sometimes that is right. Often it is not.
Let's find out which one you are.