When you wash a car with hood vents, if the water goes through into the engine, does that really damage it? If so, how do you prevent it?

Just don’t put the water where rain wouldn’t go. And don’t put the source of water directly in the vent.

The only damage it will really cause is that the electrical connectors will rust, but electrical connectors on the outside of the vehicle usually have seals and electrical grease in them to prevent that, I wash the engine bay of my car every now and then, just be sure to try and dry it off well, a air gun will work well for this as long as you don’t go full blast. Also, just watch that you don’t spray water into the intake, engines dislike water…