Question Shader either does nothing or covers everything up with grey, whats wrong?

Above you can see my render pipeline asset + the shader giving off the wrong effect.
In the full screen pass I am applying the material made with the shader below (i'm just trying to see any change on the screen, like a "hello world" kind of shader).


If I change the injection point, it just covers various layers of the game with grey.
My aim is to have the entire screen changed by the shader, like in this case study:

Not sure what I'm doing wrong. My URP asset is set correctly in graphics settings, on the camera, it has a renderer. If URP wasn't set up correctly, I believe I wouldn't be getting the grey texture covering the screen at all.

Would love some help/explanation as to what I'm missing.

 

Ah, forgot to add a screenshot of what the scene looks like without the shader:

 

TY very much for your interest, that checkbox was indeed unchecked, although it didn't solve the problem, is there something I need to do to "reload" the scene or something like that?
For future reference, my pipeline asset below:

 

I see I made a little mistake now, your previous suggestion to check that "opaque texture" box did alter something after all -- the shader now covers everything with the camera's background color rather than simple gray:

Per your suggestion, I believe this shader should be sufficient to test things out, because it'll show 4 airplanes, right? (Just plugging the base color into scene color has the same effect, just checked.)

 

Nope, I'm just getting the solid camera bg color all over the screen. I should be seeing the F-16 fighter you see in one of the screenshots above copied 4 times.

-- I recorded a quick video of what's happening, when I set the injection point to earlier, the shader stops covering it and simply appears to do nothing at all. When the injection point is later, the shader just covers everything with green. No tiling in sight.