For some reason most sites choose to leave the projection plane out of the visualization of the view frustum. The focal length often labeled "g" is the distance from the camera to the projection plane. The projection plane (which is blue in the attached image) is almost always placed at a distance from the camera such that the distance from the center to the top and bottom is 1. This image from the fged website shows it well: This choice is a matter of convenience since it makes the math all work out nice and neat. As you can see the near plane can, and often is, behind the projection plane. (the green plane in the image is the near plane)

For your second confusion: One goal of the projection matrix is to force coordinates with a z value less then that of the near plane to fall outside of the near clipping region which is -1 in OpenGL. So any coordinate that has a z value less then near value will result in a z value below -1.

For some reason most sites choose to leave the projection plane out of the visualization of the view frustum. The focal length often labeled "g" is the distance from the camera to the projection plane. The projection plane (which is blue in the attached image) is almost always placed at a distance from the camera such that the distance from the center to the top and bottom is 1. This image from the fged website shows it well: This choice is a matter of convenience since it makes the math all work out nice and neat. As you can see the near plane can, and often is, behind the projection plane. (the green plane in the image is the near plane)

For your second confusion: One goal of the projection matrix is to force coordinates with a z value less then that of the near plane to fall outside of the near clipping region which is -1 in OpenGL. So any coordinate that has a z value less then near value will result in a z value below -1 after projection.