This comes up a lot, so here are some graphics I came up with to help explain. (And I'm truly not being snarky - this is something I learned after I went through my own pregnancy, not ever being told that my boys were monochorionic, and wondering even after they were born if they were identical.)
in the second picture I say (probably) because 1 in 9 dichorionic twins are actually monozygotic, meaning the egg split early enough that two chorions formed.
Here are some real examples of early ultrasounds:
Hope that helps!