"what sorting layers it affects" might be difficult.
On the gpu level, there are no sorting layers, draw commands(drawcalls) are executed in order one by one.
If you want your blur to affect only certain layers, you would either need to execute it after certain draw calls(that belong to the affected layers) have finished, but before other draw calls, or use some kind of mask or stencil buffer that would hold the info about what pixels should be affected.
The former scenario is basically impossible due to various complications(then need to stop a rendering pass in the middle, reconfigure for the blur pass, then revert back) that unity imposes(for good reasons, like performance).
The latter is the common way to do something like this. You'll need to setup a custom render pass that runs on the specified layers, rendering the objects to a mask(a simple shader that write white or something), then another pass that renders your blur object. It should read the mask and apply thr blurring only on pixels that are in the mask.
#Need help blurring everything behind a sprite (2D URP)
green linden
Explaining in more details is gonna take a lot of time, effort and text, so I'm not gonna do it. You'll need to research these things on your own.
But if you have specific targeted questions, feel free to ask.
green linden
By reducing the resolution of the sprites?
green linden
Did it allow applying to certain layers only?
I see.
From your initial message I though you want it applied to a certain area of the screen("behind a sprite"). If it's full screen, then indeed there are many other options.
Did built in post processing really allow per layer effects though. I can't remember there being such a feature.
Or did you create multiple cameras with different PP settings applied?