The manifold always has lower pressure than the intake. But if there's a lot of blowby at WOT, that overwhelms the flow thru the PCV valve. In that case the flow direction reverses in that breather tube.

With a turbo it depends on where the breather tube is connected. One way is to connect the breather upstream of the compressor. That way you prevent pressurizing the crankcase. Under boost the PCV valve closes & whatever blow-by you have, flows into the intake. Yes it gets some oily mess into there, but at least it doesn't blow seals out of the crankcase.