Books/publishing deals. Where is the money made?

Curious about the business side of book deals. I listen to a bunch of history podcasts and nearly every person interviewed is plugging their new book, but these are often super niche topics (fashion trends in Victorian England, etc). I’m sure it’s the same for many other subjects.

Do these authors really make any money off of these deals? Are they subsidized somehow so they aren’t dependent upon actual sales? What’s in it for the publisher if there are super limited sales (as in, does the publisher make enough to justify doing the publishing)? Just sheer curiosity on my part. TIA