Some people think you should give way to vehicles approaching from the right on a roundabout.
But, that's not the rule.
You must give way to any vehicles already on the roundabout before you enter.
So if a car already on a roundabout just happens to be on your right, you must give way to it. Here, car A must give way to car B.
And if a car already on a roundabout happens to be directly in front of you or on your left, you must give way to it too. So, once again, car A must give way to car B.
It doesn't matter if a car is on your left or on your right, if it's already on the roundabout, you must give way to it before you enter.
Giving way on roundabouts is straightforward, when you know the rules.