How to create a polygon primitive between two highlighted edges? They have different polycount.

Add each edge loop to it's own group.
Merge the mesh together.
Then drop down a PolyBridge node.
Populate the two source and destination groups with the groups you created from the edge loops.
This will create a bridge of faces between the two loops.
Follow up with a Facet node to uniquify the points, this will fix any normals issues.

Another approach would be to resample one curve so that it has the same number of points as the destination. Then the PolyBridge should make an all quads connection, otherwise you will get some triangles.