I need help with a special kind of Cave

So I´ve been wanting to create very spacious caves and I looked a bit into the Primordial Caves from Alex´s Caves (which are what I´m thinking of) and I found out that there is a feature used, I also know how it would work but I can´t find a way what kind of feature it would be or how it could be made. As far as I know it would be either a custom air structure or a big blob feature replacing blocks with air. Does anybody know how I could make something like that?

(Included img depicts the Primordial Caves)