How do TWIC, chessgames.com, chessbase, etc. keep up to date with new games? Most players use these services to keep up to date on the latest chess results, but how do these services source their databases? It would seem like a prohibitive amount of work to scrape results from various tournament websites, so is there an official source that everyone uses? If it's FIDE, I wasn't able to find it on their site.

Tournaments often have a place to turn in your scoresheets, but what happens between that and the game showing up in one of these databases?
There's no official source for that. Someone just has to download the PGN games manually.

It is indeed a prohibitive amount of work, but we are talking about money here. They are not doing the collection for fun, they are trying to sell their services for something else such as paid membership or book advertisements.

Let's take a look at the TWIC homepage. Please note the paid ads by New In Chess. They're offering you PGN download as a business.