The seat is like a carseat, but isn't very big. I used in back in February with my 9 month old and she pretty much filled it, so I think a 20 month old would struggle in it.
In club suite the carry cot/seat is attached to a shelf at the front left or right of the cabin. In club world (old) its attached to a drop down table at the rear centre pair. They are designated bassinet positions in the seat map.
You can't use the seat for taxi/take off/landing, they need to be strapped to you. They're only really useful for them to sleep during the cruise.
Thanks for these details - this is very helpful to know. My hope was that we'd be able to use the infant seat during taxi, takeoff, and landing since at 20 months old, they can be challenging to keep in your lap for long, so I appreciate the info!