I'm working on a novella where the protagonist finds himself in a new world in Chapter 4. How do I avoid infodump and maintain the reader's interest whole furthering plot and theme? The time period is about the same in both worlds, and the new world is a reflection of the first with some major changes in politics, economics, human relationships and architecture.
If the protagonist doesn't know the world just as much as the reader, then you're in an enviable spot; it means that practically every revelation about this 'new world' is going to be shown to the reader as the protag discovers it.
In essence, just have the protagonist organically discover the ins and outs of the new world through exploration, asking the right people questions, meeting and getting to know other characters, etc, etc.
Having an outsider discover a fantasy world rather than pre-existing in it is one of the most narratively convenient setups (it's one of many reasons escapist fantasies often use the setup)
Think of this challenge like taking a photo. Your character is your lens, the character should be the vehicle for capturing everything you see, through how they feel and experience your environment. That said, if you show a photo to someone, you may tell them how you took the picture while at the beach with your family, and that your dad is drenched because he slipped when walking across a small pier and mom thought it was so funny that, that was the moment she wanted for this years Christmas photo. That bit of extra context can come from you, but other then some minor context, your reader will be more compelled by your world when it’s personalized through your character.