The speed of light is fixed. So when we look at light that is emitted from something that is moving away from us very fast it gets red shifted. When it’s moving towards us it gets blue shifted. Same idea if we’re the ones moving and seeing.

Also because light moves at fixed speed the light we see that’s far away was emitted a very long time ago. So we see things as they were when the light was emitted.

We can’t see infra red, but our cameras can. And we can make sensors that can see much lower frequencies than that. However some stuff is so far/long ago away that it gets shifted beyond any ability to measure and becomes cosmic background radiation that essentially acts as noise.

Some things will simply never reach us as long as the universe is expanding, and somethings are so shifted that they cannot be observed meaningfully.

We can infer a lot about the parts we cannot observe because there is a general spherical symmetry to the Big Bang.

But the edge is just the farthest anything from the Big Bang has gone that we can observe . We haven’t seen/understood evidence that there are things that weren’t from the Big Bang yet.