Beginner Questions - Watching with Envy

Okay, I have been popping in and out of this forum for almost 6 months now. I am so in awe of so many of your creations. I am trying to get up the courage to do some of this stuff and I have so many ideas. I want to make stones, spheres, garden decor, rocks, ponds...the works. (I really jump into stuff head first) Anyway, just wanted to ask a few questions.

Why hypertufa? Is it because it is lighter? Why use this as opposed to regular concrete? Or does which one you use depend on what you are making?

I see so many finials, spheres and things I love but the molds are so expensive. Most are $400 and up for some of the stuff I want. However, it seems to me, some of the molds are geared to commercial businesses. Isn't there a cheaper way to get say, something like the pineapple finial things for the everyday layperson?

When you want to make things like large boulders (the rounded, huge ones) I would guess you have to start with something underneath. Can you use styrofoam? I saw it in a pond post here? Would it really matter what you used underneath to start your creation if it is all covered in cement?

(please go slow, I am blonde and have never worked with cement before.)