Domains of language are arbitrary. They can be broad and narrow. Books and movies are different domains, fiction books and non-fiction books are different domains, Drama and comedy are different domains, mystery anime and slice of life anime are different domains, and so on.
In practice language domains are treated similarly to genres. As language learners we often want to tackle one domain at a time in order to increase comprehension, speed of learning, the frequency at which we come across domain-specific vocabulary, etc. The smaller a domain is, the quicker you can master it.
But don't pay too much attention to domains. Being strict about sticking to a set genre or domain can lead to boredom and eventually to burnout. You've got to find the right balance. The key to succeeding at learning a language through immersion is having fun. Can you really imagine yourself having second-thoughts like, "I really want to immerse in this content, but I can't because it belongs to a different domain."? Just immerse using the content you like.
Having said that, domain boundaries can pose challenges to beginners. Every time you switch to a different domain, your comprehension drops. Each new domain contains new, unknown vocabulary. For the first few months try to keep your domain narrow. In the guide I recommend watching only slice of life anime, for example.