A Mushroom Vent Cowl is a domed plastic fitting which allows air to escape without letting water or debris in. From the top the domed surface allows water to run off of it, but from the bottom it attaches to a 110mm push fit pipe. There’s a gap at the bottom of the dome which will allow air to escape.
Usually the vent is used at the top of the soil stack, above roof level, where it will allow gasses to escape without the smell being noticed by anyone else. The mushroom vent simply pushes over the top of a plain ended piece of
110mm soil pipe, However a solvent weld cement can be used to glue it on for a more secure fit if required.