I'm a stone-cold amateur at woodworking. Doing my very first project
of any kind: making a simple desk out of some plywood and electrical
conduits for legs.

I am sanding the plywood (birch) and I realized that I don't know when
I am done... I know that I am going to add polyurethane after this,
but am unclear whether that is *only* for keeping moisture out... or
if that is also the final stage of "smoothing."

Do I sand until the surface of the desk is perfectly smooth and then
add the polyurethane? Or is mostly smooth okay and then the
polyurethane makes it smoother?

In general, how smooth can I expect to make the surface of my desk?
Can I get it as smooth as the ones in Home Depot? Or are they that
smooth because of some other material/procedure?

thanks for any help!
scott.