As for being taught: it was taught by both my driving instructors, in the sense that I was consistently told off for opening my door using the wrong hand, even if I was very careful to check my mirrors and side. I wasn't asked to perform this feat on my driving test, but they don't need to be explicit: it's fairly challenging to exit the vehicle without opening the door.
When you open the door you grab the door handle -- the thing that opens the door -- with your right and plastic black handle -- the thing that gives you more 'grip' -- with your left. By doing this you have a lot more control on closing the door as well if you still happen to accidentally open it when you shouldn't have.
I had to retract once, because it appeared I had a blindspot where I couldn't see a cyclist.
As for the whole helmet thing, they do not make you more safe. Proper infrastructure makes you more safe. Unless you're riding downhill, racing or performing stunts. You don't wear kneepads when you walk down the street do you?
Like in the Netherlands, bike lanes in Denmark are mostly placed between the road and the side walk, so by not looking out for trafic you risk getting your door torn of by an SUV, rather than hitting a bike.
In rural areas where bikers ride on the sidewalk and are not super common, sure, but I can't remember a time driving in a city where I was NOT super paranoid about hitting a car or a cyclist or a pedestrian with my door.
I really like my door not to have dents, scratches and blood all over it (especially mine) so I look in the already-optimally-positioned mirror on the outside of the door where my hand goes to check if there is some other vehicle coming first before I open the door.
And I am not even Dutch.