Could someone clarify how water from the battery tray could cause significant water collection inside car?

On my w115 220D, the battery is in the engine compartment up front near the headlamp. I am not familiar with other vehicles, so maybe that is what I am missing.

I have a similar problem as the OP, and I know my battery tray is rusting out. But I am not sure if these are somehow related. Thanks.


EDIT FROM 10 MINUTES LATER: Nevermind, I finally finished watching the video linked above and it answered my question.