First picture: go to top view, and looking down the length of the cylinder select half the vertices. Delete them (press X), then rebuild the faces on the exposed side (select four vertices and press F).
Second picture: use an uncapped cylinder. Select all vertices, extrude (E) then right click so you don’t move the new vertices. Press S to scale, then Shift-Z (to scale only along the x- and y- axes) and scale the interior down. Left click to finish.
Hope this helps.
Cut the cyclinder in half and duplicate it?