Grease pencil keyframes

hello there,
can someone explain why we do not use/see diamonds keyframes markers when doing 2D animation in blender grease pencil?
after donwloading and examinating samples .blend files from https://studio.blender.org/training/grease-pencil-fundamentals/5c4a094779f30a170cb9032f/ or cloud.blender.org (like hero) we ca push the play button to see and render the animation. The dope sheet remains empty. No keyframes.
for example, the 2d animation cowboy shooting in the sky has invisible keyframes, but it is animated for 100 frames.
thank you