Supernovae are said to shine brighter than whole galaxies, but how is that determined? How is "brightness" measured in astronomy?

If a galaxy is already super bright, then how do we know that a supernova shines brighter? I have seen examples where a supernova towards the edge of a galaxy looks "obvious" since it appears as a bright dot.

But the edges of galaxies are not as bright as the center, so this is simple to "see." But what if the supernova happens near the center of the galaxy? Can it still shine "brighter"?

When does it make sense to even use "brightness" to describe objects in space?

At some point, our eyes can no longer distinguish between two things that are extremely bright. Of course, I'm only thinking about visible light.

Thanks in advance for the answers!