In-Universe example: Nigel and Marianne spend one episode trying to film the bird-eating spider. However, Marianne is frustrated at how many of Nigel's turns appear to be false, instead leading them to other prey-species that are apparently dead-ends. However, Nigel points out that the Bird-eating spider rarely eats birds (Which is actually true!) and that by finding the species that the spider primarily dines on, they'll be much more likely to find the spider than if they followed birds.
So Eliza can talk to animals and just generally understand their speech, and her parents film a nature documentary. What happens whenever they film footage of a predator killing and eating it's prey? Imagine what something like, say, the sea-turtle hatching season - in which a LOT of the baby sea turtles get picked off by birds and crabs and other predators before they even reach the sea sounds like to her? Of course she rarely sticks near her parents when they are filming but she must have experienced this sort of thing at some point in the wild just by happen-stance anyway.
Mating season, both in the wild and in nature documentaries, as heard by Eliza. Imagine walking through a forest and suddenly you hear what sounds like a porno, made even more awkward by not being able to tell if it's coming from humans or animals, since animals sound like humans to her.
Debbie's attitude likely stems from a form of deep social isolation. She's often on her computer, secluded from her family, but that just means she might be chatting with online friends or absorbed in her own hobbies as means of distraction from loneliness. She had to grow up constantly moving around from what can be assumed to be a young age, meaning she was never able to make any long-term real life friends or socialize in a normal way. On an even less pleasant note, she also had to go through puberty and have all the urges and needs of a developing teenage girl and essentially zero privacy since she lives in an RV with her entire family. Her parents love her but have basically denied her any sense of privacy, personal space or control over her own life. No wonder she's moody and has little interest in people.