The tablet case has 2 pins that fit into the keyboard. See picture 3. It holds great! Also on the knees :)

I have since bought the keyboard and the tablet (Samsung Galaxy Tab 6S lite). The tablet holds very well in the keyboard. The connection is actually magnetic (the two pins only serve as additional stabilisation) and really stable. What I was looking for.
2 weak points: (1) The holder of the pen is somewhat superficially solved. When opening or closing the keyboard, the pen is often touched by the lower edge of the tablet and pushed away. The pen then hangs somewhere on the tablet (because it is magnetic). So it doesn't get lost, but it's a bit weak. (2) I find the overall finish of the keyboard a bit (somewhat) cheap. I don't think it's very durable. I'll buy another keyboard in stock. And when closed, the keyboard and tablet move towards each other. I still need to find a case so that it doesn't open in my daypack and other things slip in between. I think that's a small thing, but it's really weak.