Engine oil under pressure enters the bearing housing of the impeller shaft from the oil inlet side. This oil passes bearing and the wheel journal surfaces lubricating and cooling the moving components. Here it depressurize and flows by gravity through the oil drain line and into the engine sump.
Seals are fitted at both ends of the impeller shaft. The primary purpose of these seals is to seal the high pressure gasses (exhaust) or air (intake) from entering the center housing and into the crankcase. The secondary purpose is to prevent the oil from entering the housing.
The image shows how the oil is returned from the turbo to the engine sump via gravity. The braid protects against the heat and the AN fittings suggest that this is an upgraded part, whereas a typical stock part consist of hard line.
If the oil return line from the turbocharger is blocked it raises the oil pressure in the turbo charger which then escapes past the oil seals at either end. The oil then enters the intake tract where it is sucked into the combustion chamber or it is blown out the turbine side into the exhaust.
The oil return line must always be free so that an unpressured return of the oil can be ensured. The oil return line can get blocked by becoming too hot which then makes the oil coke in the line. The overheating can be due to poor quality oil or poor cooling of the engine as a whole. Other reasons can be incorrect routing of the turbo oil lines (too close to the exhaust manifold), non-insulated lines or incorrectly fitted/missing heat shield.