Hi there. I do understand your question, and yes it is mind bending !

The inflation of space that occurred instants after the big bang was much much faster than the speed of light. So the light emitted from a given position could only reach a small fraction of that space. It describes a sphere around that point of radius (speed of light x time).

Today, this radius increases with time, but so does space (the universe is still expanding ).
In the same way, the light that reaches that point has covered the same distance, that is the radius of the sphere. So what we see from that point is light that was emitted from the surface of that sphere.

One question that I have is, is the CMB supposed to be a background signal originating from the edge of the full universe?
If so does it mean that our horizon is'close' to the edge of the universe?

The CMB corresponds to the limit of the volume of space we can observe, the Universe being measured 'flat' implies that this volume of space is most probably minuscule compared to the whole universe. This is actually the center of my argumentation about the potential illusion that is the matter/antimatter unbalance: we just cannot see far enough to gather enough data! Check out the next episode (it's just in my head right now, but it should be quite mind boggling too!).

I hope this clarifies things,

Be well