How are ground wires and OPGW connected to ground and neutral point of transformer, and to the com.system?
The ground wires are not connected to transformer neutrals.  They are typically connected to the steel towers along the transmission line.  At the substation ends of the line they are connected to the sub ground grid.  There are variations in practice from utility to utility but none that i know of try to directly connect to the transformer neutral.
The transformer neutral impedance connects directly between the star point (neutral) and the substation ground grid.  The overhead ground wires (OPGW or anything else) are tied to the ground grid, not the transformer star point.
The idea of the overhead ground wire (static wire or shield wire) is lightning protection on transmission lines.  Distribution is a different animal and practices vary widely.
Here is an old good post by @Transistor on this topic, and here is a paper that discusses the various approaches to use of these &quot;shield wires&quot;.  The end of this video shows that the OPGW is brought down the tower (in substation) and the fiber is connected to an interface box (for isolation).