What are these parts called?

What are these parts (with rust forming) called? I'm not very car-savvy. Just got a new Toyota Tundra a couple months ago and had the dealer do an undercoating for rust protection, but it seems like either they missed these parts or intentionally avoided it. Is it because you're not supposed to coat these parts? Are they cheap/easy to replace eventually? Or should I try to remove the rust and apply something like fluid film on there? Thanks.