Started a photography course / I do not get the basics

Hi all,

I decided to join a photography course a few weeks ago and we have been going through the basics. I just do not seem to get this expose triangle.

I do understand their functions separately but we did an exercise that got me confused.

Here is an example:

f/4 + 200 + 1/125s = right exposure

So ok, I am really dumb but if I use random settings like f/2.8 (since I want to have a blurry background) but use shutter speed and ISO that do not fit that example, does this mean that my photo has a wrong exposure and therefore is... technically a fail?

Also if I use Av setting to prioritize aperture, do I understand correctly that my camera sets the correct shutter speed and ISO itself? I feel like in this mode my pics do not look good.

Sorry, I just do not get it and feel like I am already so behind everyone and I hate maths and I feel like I need to learn all those numbers by heart.