If I understand right, you want to connect two objects, for example, a disc shape and a tube shape…
The first way is to just do it by hand, modeling, stitch and sew, polypen etc… since the point counts don’t match it could be involved. The first step is to select both objects and combine them into 1 object. Then move on to welding the points together.
Another way could be to use the volume mesher, add both objects and move them into place, this will mesh them together, it will create a high poly mesh but you could use the Z remesher. It may get you close to where you want to be.
Another way, although this would only work in some situations, could be, to extrude your ‘mask shape’ then use that to boole the second shape so the cut takes on the masks form. But again that is likely only to work in limited places!
Share a file and I’m sure someone can help
Cheers
AK