a bivvy bag is a waterproof sack that your sleeping bag goes in to keep the rain out.
you can get hooped bivvys which are like bags with hoops over your head. you cant sit up in these
you can get double hoop bivvys which have hoops at top and bottom.
the difference between a bivvy and a small tent is a bivvy is one sealed unit.
in ops picture, the so called bivvy is actually a tarp like tent that is strung out over an internal net structure.
this pic is a bivvy because its still just a sealed bag even tho it has a hoop.
I am noob also but this is what ive picked up