In a modern car the engine bay itself functions as a duct/conduit. In most of these systems overall, the reason for ducting isn't to contain where the hot air itself goes, but to create an airflow environment that allows the fan/radiator system to operate most efficiently - to make a situation where the air gets out as quickly as possible. Blowing the hot air back over the engine is fine as long as it has somewhere to go ASAP so more fresh/cold air can enter the system.

In most factory cars this is achieved by negative pressure in the engine bay, through the use of undertrays and cowl design.

In race cars you will often see a more direct exit, the most common in front engine cars being a V-mount radiator / intercooler setup with one or the other vented directly through a duct in the hood.