Say that you shifted into a Giant Spider, do you shift into that SAME giant spider each time you shift into giant spider form? Or can you control distinguishing features like "a heart is on the Giant spider" or something.

You just change into the shape of an everyday giant spider.

The everyday spider still has distinguishing features, like a missing leg or a slightly longer fang.

In fact, I am sure if you looked long enough, you'll find a potrait of Jesus on one of them.

Well, you look exactly like what is represented IG.

Always remember this rule of thumb in these sorts of cases:

"What you see is what you get."

IE, if you see a paladin with a glowy sword, you can assume that shit is going to hurt.

If you see a polymorphed wizard as a troll, you don't see someone polymorphed, you just see a troll.

etc.