If you are referring to the Diamond Princess flights, I believe they came back on two special chartered 747 cargo converted to passenger, with special quarantine setup. No regular commercial passengers exposed. Somebody somewhere in in US Gov surprised me.
It’s the same set up they use to move troops to the Middle East.
OK... I think that you are right as I was just reading some more. Instead it was a mix of those known to have the virus, and those that are unknown. But anything that is airborne will be carried through the whole plane. You would have to have a very specially built plane to stop that. And I don’t know if such a plane has ever been built.
And pilots should now be put under quarantine as well.