How do shotcut masks work?

How do shotcut masks work?

I have an image that contains a green circle… With the chromatic mask filter I thought that the green circle would become transparent and let the video appear below in this circle…
But no, nothing works… all my images containing green (or other color) leave the green shape displayed and the rest become transparent.

When I watch the videos on the subject, the mask reacts like an alpha channel… So what’s going on!
Shotcut 22.12.21
Thanks for your help !

Mask = Alpha Channel

Since you mention “below” maybe you only need a Chroma Key filter and not Mask.

Oh yes ! I confuse ! the term mask in French is different from English!
Chroma Key filter should work - thanks