These items are made to fit to standard 140mm fans commonly made by computer, chassis and rack fan manufacturers to connect to a 100mm flexible ventilation pipe with a clamp.
There are two different looks available. The basic versions use much less plastic. The extended version provides a longer graduation from 140 to 100mm which may or may not be better air flow wise.
There are also two sizes of each design available. The short variant has a 20mm section to clamp the pipe, the long version has a 40mm section for clamping which is easier especially if you do not cut your pipe exactly evenly, but uses a bit more plastic.