Air to belly?

Does any air get in or out of cargo hold at constant cabin pressure?

The belly hold must be pressured, because otherwise the floor of the cabin could not bear the air pressure. Thus, air has to get out of the hold on climb to cruise and get in during descent. But does any air move during the cruise when constant cabin pressure is kept?

If a fire breaks out in the cargo hold, does it fill the cabin and cockpit with smoke? Or will the smoke just stay in the belly until the fire consumes all oxygen in the hold and goes out?