In the above links, we studied these actions using some illustrations and also studied some specific variations of these actions (e.g.) short recoil action, long recoil action, direct gas impingment, short stroke gas operation etc. We also studied examples of weapons that used these different actions.
Thanks to the efforts of the US Army, we actually have a movie that illustrates the basics of all of these actions.
The video clearly illustrates how the various actions work, far better than static images do. Happy viewing!