When talking about a plate, are we talking about 15kg or 20kg per side?

45 lbs or 20 kg

when doing barebell row do I have to be perfectly parallel to the ground or is it okay to have your body slightly up?

That just a different balance of trap / lat work but if you're trying to do Pendlay rows try to keep it horizontal.

Also wondering if it is bad that my hands aren't really straight when pulling, is this bad - should I work on it?

Don't follow your questions

I've been told that taking my shake right after a meal is a bad thing. Is this true? Shouldn't there be no difference in which order and when I am getting my macros? 100g protein should equal 100g protein, right?

Shouldn't matter really but I do try to spread my protein out through the day just in case there is some absorption limit.

is it true that different exercises will change the way your biceps will develope? I've been told that some exercises will make your biceps bigger (as in higher), while others will make it wider.

To some extent yes, but they're not mutually exclusive. You're talking more advanced level bodybuilder type sculpting getting that detailed.

when doing bench I feel it way more in my shoulders than in my chest? What could be possible reasons for it? Are my shoulders just not developed enough to actually stabilize the weight and need to work more?

Most commonly that's from not retracting the scapula.

can anyone recommend a tutorial for how to bench perfectly which doesn't include arching your back? It just doesn't look healthy to me and I've never seen anyone in my gym doing it which is why I would feel uncomfortable + I would have no one to ask to check my form.

You don't need a ridiculous arch, just a little. For all you know the people at the gym are doing it and their shirts wouldn't let you see it unless they're wearing spandex.