View the body against the amour and adjust the vertexes to increase the area under the armpitt.
Select the faces in red.
Extrude as opposite.
Weld with Weld Target the 4 vertexes at the bottom to have a connection as shown in the image.
Finally apply Make Planar to the faces to have plane surfaces.
Simplify the geometry by welding the interior vertexes as shown opposite.
Insert a series of edges in front of and behind and move the vertexes to make an edge (use move with local axis selected)
Move the edges in red towards the outer edge as shown.
For the armour of the arm, start from a primitive cylinder with 8 dimensions, apply a rotation of 22.5° and adjust whilst compared to the arm like above.
With a uniform scale, adjust the size of the face in red.
Make a series of Extrude/Bevels like opposite.
Adjust the new polygon as shown.
Another a series of Extrude/Bevels. Adjust the resulting polygons once more.
In the same way make these two extrusions.