Thank you all for your comments.
I know it is a simple and not sophisticated pond. Easy and cheap to do.
Does not have a skimmer but it takes less than 5 minutes every day cleaning the surface by self made pantyhose hand skimmer.
Also no bottom drain but because the pond bottom has inclination and all the debris get collected to the deepest part and easy to suck out.
The aeration is made by the waterfall and by a simple aquarium pump added on the hottest days.
Also liner is much practical than concreate and other stuff, in my opinion, because if something goes wrong you can easily replace or just lay another one on the previous.
But a nice bog filter would be very nice if I had some more place.
Pond+lilies+rain is another beauty