What can be seen under the 'bin bag' is the arrangement of the pressure hull and the outer fairing around it. These days, internet searches give AI explanations first, but from that:
"The Dreadnought class, like many modern submarines, utilizes a double hull structure. The outer hull, or casing, is designed to be hydrodynamic and free-flooding, meaning it is not pressurized and allows water to flow through it..."
A lot of free flooding space then!