How do the Death Stars retain atmosphere in the landing bays?

In episodes IV: A New Hope and VI: Return of the Jedi we are shown ships landing in Death Star 1 and Death Star 2 landing bays seemingly exposed to he vacuum of space, and yet non-suited Imperial personnel are seen standing on deck (for example in about the 3rd minute of Episode VI). How does this work in-world?

If there are some kind of shields, what happens to them when the ships cross the threshold? Why don't we see de/repressurization?

What, in short, is the deal?