system
Given the following post, respond as if you're commenting on Reddit:
human
[The Expanse] What's going on in the rest of the Solar System?

I'm using the TV series as my basis for this as I have not read any of the books. 

The action and intrigue seems to focus solely on Earth, Mars, and the Belt. What about the rest of the Solar System? Does humanity have a presence beyond the Belt, on moons such as Europa and Titan, which I would imagine are prime candidates for colonization? 

We have the Outer Planets Alliance, which I would imagine encompasses everything that's not Earth and Mars. What about Venus and Mercury?
gpt
~45 million live around Jupiter, with the largest colonies on Ganymede and Callisto.  Ganymede's magnetosphere helps shield it from radiation, making it a prime location for growing food and having children.

The Saturn system has a population of ~20 million.  Titan is known for its amazing dome resorts.  Phoebe is a point of contention because Belters aren't even allowed into orbit due to the secretive nature of the research outpost there.

The furthest outpost is a small science settlement on Titania, around Uranus.  Population ~5000.

Mercury and Venus aren't populated.  Venus remains a hellhole, Mercury has nothing in particular to make it worth the effort.