[quote=@Light]
When it comes to being from a plane/world other than Earth, we can just make up our own shit more or less, right? 
[/quote]

Yeah, think of it like a multiverse. Each Mortal Plane is like it's own universe existing in the same space as other universes but at different vibrational frequencies. So races, culture, level of technology, if there are multiple planets accessible on that plane, what those environments are like, etc. are all in your hands. We don't need to set a predefined amount of planes - we'll just say there are "a lot," lol. At least several dozen. 

For instance, one of my characters will be based on Ian Lightfoot from Onward, and he comes from a plane that has lots of magical creature running around, but magic itself is fairly rare there, so him going to Qrowdell is a huge deal, especially to his older brother, who thinks it's the coolest thing ever that they now have a Magi in the family.