Help me Make Sense of Unity 2D Resolution Management

I am trying to set up an environment for a retro style pixel art game in Unity 2D. I aim for a vertical resolution of 240 pixels, and then, I would like the horizontal resolution to be dependent on the target screen’s aspect ratio, so, on a 4:3 screen 320×240, on 16:9 around ˜432×240, and so on.

I have read a bunch of tutorials, added pixel perfect to my main camera, set up a target resolution in player preferences, set the pixel in a unit to 8 pixels everywhere, but still cannot make heads or tails of how it actually works.

Here is an image of my scene with my camera selected:

enter image description here

If I am correct, the dashed line of the camera is my target resolution, although it is weird as the green image I used as a background has a native resolution of 216 pixel vertical, yet it fills up the supposedly 240 pixel high camera. I have no idea what the solid green line is, and don’t even get me started of the canvas, whose render target is set for camera yet it is obviously many times bigger than that.

Can someone please explain it to me how to properly set up a 2D environment described in my opening, point out my mistakes, point me to a good tutorial, or just give me advice on how to understand Unity’s resolution management?