There are an infinite number of universes, and they can be startlingly different -- from culture, to cosmology, to physics itself. Magic or Technology may be plentiful, rare, or basically absent; and occasionally physical constants and major forces of physics may be a little wonky in unexpected ways (sometimes to the detriment of matter as we know it). Most universes, however, manage to support life in one way or another- and quite frequently, that life includes humanity.
All known universes have at least one connection to another known universe (obviously), and an improbably large number of these connections are to the dimension known as Cross. Cross consists mostly of a single gigantic city (Portshire), divided into walled Districts. Many districts have extreme variance in social rank, profession, universe of origin, tech level, magic level, species, religion, and etc from one block to the next, unless the district has spent time and effort to specialize.
"Weather": Speaking of which, the more influential and organized districts often find ways to encourage physics to act how they like within their domain : usually by taking advantage of the fact that magic levels and other localized rules of physics tend to leak through open cross-dimensional portals. People are still learning to track and control physics leaks of this kind, which can flow unpredictably from place to place, and are colloquially referred to as "weather". Poorer districts and those with lax rules about portal placement often have very little control over the "weather".
The Fog: Outside the city and in carefully-walled off areas of varying size, there is a kind of "weather" known as The Fog. It is misty and obviously cloud-like, comes in various colors, and usually people who go in do not come out. Sometimes they come out before they went in, or several instances of them will come out (all with different traits, some of whom cannot see eachother). Sometimes people come out that never went in. Things that try to exit the Fog are known as "fogborn", regardless what they appear to have been before. Many (but not all) fogborn eventually prove to have unsettling properties that were not immediately apparent. Most districts do not allow them inside. ReynoldsDistrict is an exception.
Governance: Districts and Interest Groups have representatives. Representatives form councils, which make semi-local decisions. These local councils send their own representatives to higher-tier councils, to make even bigger scale decisions. It goes all the way up like this, until eventually you get to the top, where the Portshire Council makes decisions for the entire city. They accomplish basically nothing, unless you come to them with a Yes Or No request to either approve or veto (much like the Citadel Council from Mass Effect). Representatives of all levels are not allowed to leave Cross or risk any contact with the Fog, or they will lose their position.