How do you clean the keyboard of your laptop / computer?

Turning it upside down and shaking gently will get some of the dust and muck out but a blast of compressed air (you can buy cans of compressed air or you can use a vacuum cleaner if it has the ability to change to nozzle from suck to blow) does a much better job. To clean the surface of the keys and the body of the lap-top, use a clean, lint-free cloth just moistened with plain soap and water or use a proprietary screen cleaner applied to the cloth, not directly to the screen or keyboard.

I have opened the keyboard to clean, maybe this is not simple and I have to use a vacuum cleaner, but I chose to open it because I want to know what's inside

I do what you do but I have a little brush that I sometimes use as well.

I use a paint brush

I have a brush that I use to clean it

The same way as you do..it's better to use ear buds to clean the dirt between the keys.

a toothbrush - one you don't use of course

I use a small paintbrush with hard bristles.