Need some vocabulary terms

Hello,

I'm working on a duct sealing project and I have some questions but before I ask them I need a couple vocabulary terms, which I was not able to find with a Google search.

My house has two AC systems, one for upstairs and one downstairs. Both of them have a sort of three-box arrangement. The first box is the furnace. This attaches to a second box, then this second box is attached to a third box, where all the ducts come out. What are these second and third boxes called? Are they called "transition" and "plenum"?

Almost all of the ducts are rigid, but two are flex duct for a 12-foot span or so. These flex ducts attach to a metal piece that juts out from the AC unit. This jutting-out piece is a "boot"?

TIA,
Cakemeister