The body flange and bonnet flange are joined by studs and nuts, with a gasket of suitable design/material inserted between the flange faces to facilitate proper sealing.
In pressure seal type, Bonnet takes the help of its bolts pull itself up. This in turn creates a seal between the gasket and the inner diameter of the valve body, facilitating tight sealing of bonnet against seal gasket.
In other words pressure of fluid itself is used to facilitate the sealing.