The idea is that stuff dislodged by the flossing gets brushed away and spit out when you brush. If you floss after brushing, it is more likely the floss debris will remain on your teeth. You’ve just moved it from between to a side.

That was the argument I found when I recently looked into this. It makes sense if all you’re doing is brushing and flossing. If there are further steps in your routine that would take care of the flossing leftovers, I expect the order does not matter.

I'm guessing the toothpaste is able to penetrate the gaps that were filled by debris.

I suspect it doesn't really make a difference, there may have been one mediocre study which showed a couple percent improvement, or more than likely one opinion simply gained more traction than the other.

Both orders have downsides - if you floss first, you risk forcing detritus back between your teeth while brushing.

If you brush first, you risk leaving some of the gunk from between your teeth on the surfaces - although perhaps now it will be bound loosely enough to be washed off with water.

Either way you're probably getting rid of most of the organic material anyway.

I always thought that flossing mostly moves the dirt from between your tooth to another side. So I always brush after flossing.

(I also never thought about fluoride so of course I rinse after brushing and I will continue to do so because otherwise it would be very uncomfortable)