Shrinkwrap confusion

Hi
I seem to have a misconception on how shrinkwrap works. I thought the active object(one using shrinkwrap) would use the target object vertex groups to shape it. But the targets vertex groups are never available in the vertex groups menu only the active groups vertex show in the dropdown. Is this right?
seems like shrinkwrap should need to know what part of the target object you want to wrap it round?

Is this right?
Yes

seems like shrinkwrap should need to know what part of the target object you want to wrap it round?
or what parts of the active object to shrink onto the target object

Read https://docs.blender.org/manual/es/dev/modeling/modifiers/deform/shrinkwrap.html

No, you have it wrong. The vertex group refers to the active object. Basically you use it if you only want to shrinkwrap some of the verts, and leave the others in place.

yea i guess the snap to vertex /surface command makes the need for target vertex group redundant

Vertex group and weight painting go quite well together, but you can enter sculpt mode as well while using the shrinkwrap modifier.