> First, I am trying to isolate a certain region in my raw video and make
it into it's own layer so that I can use it as my foreground. I have
other assets that I am trying to make pass behind the fg and have it go
out of view as expected. I used the rotobrush to highlight the feature,
perhaps to cut/paste into a new layer but with no success but maybe I am
going about it the wrong way. Also, after I highlighted the region and
played my video back, only the highlighted region was visible the rest
Don't think in terms of cutting and pasting.
Think in terms of laying one layer on top of another so that some layers show through others in the places that you make transparent. This is compositing.
Duplicate your layer. Use masks, the Roto Brush tool, or whatever other means to make part of the top layer transparent. Then you can put something else between the two layers and have it appear behind one object in the top layer but in front of the rest of the objects (in the background layer).
I'll leave your other two questions alone for now. You need to get comfortable with the basics of After Effects layers and transparency before you muddle with things like expressions and 3D cylinders. (You can do whatever you want, of course, but I'll limit my answer for now to the fundamentals.)
I recommend that you start here and work through all of the materials linked to:
Thanks for the advice, I will check out those tutorials.