To assemble the body, merge the head and the eye in the bodys scene.
Remove Meshsmooth from the stacks of both objects and keep only Editable Mesh.
Using the background image as a guide scale the body with a uniform scale.
On the body, erase the polygon in the middle of the neck.
Then with the Attach button, click on the head to make one object.
With Create, build the faces to connect the head to the body.
As it happens there is the same number of vertexes in top and in bottom thus one has only quadrangles with the final mesh at this join.
Finally adjust the vertexes of the neck to give a regular form.
To make sure that the vertexes are placed on the bodys axis of symmetry and so all aligned vertically, select them then select non uniform scale and then right click on the icon to open the 'transform type-in' enter 0 in the field of the x axis.
All these vertexes are now perfectly on the same vertical axis.
Select all the objects and in the Hierarchy panel, select Pivot, Affect Only Pivot and click on 'Center to Object'.
That makes it possible to center the pivots of the objects in their geometrical center.
Indeed until now the body and the head had kept the pivot of the initial starting object
It is easier to move the right part of the character after the copy to mirror in Copy mode
Select the left part and with Attach activated, click on the right part.
Select the vertexes of the body's axis of symmetry
To weld the two parts it is enough to apply Weld with a low value.
This value indicates a distance to which vertexes within this value become welded together.
To be sure to weld the vertexes in their pairs check that the number of selected vertecies after Weld is half of those
If it is not the case, decrease the tolerance of Weld etc...
Here, max indicated 166 vertexes before Weld and 83 after thus they were welded in their pairs as desired...
Check visually that they are all correctly welded by looking at the model in low poly.
Then apply a Meshsmooth modifier.
As usual, preserve the low poly model for the next parts (dont collapse!)