12:54 AM, Wednesday December 29th 2021
Firstly, this is not easy. You are training your brain to be able to imagine how things interact in 3d space.
The objects can be interpreted in different ways but mainly in which object is in front of another and this will dictate which object is being cut into and which is doing the cutting.
In the case of your second one it seems like the larger box could be engulfing the smaller one. When starting it would be best to start with objects which don't overlap as much as these ones day. i have linked my very rough interpretation of the forms here.
In general when I did this lesson. I tended to follow an edge into the other object and stop when the edge intersected the plane it was approaching. This requires visualisation to decide if it is convincing. I would then try and imagine the edge created by the planes intersecting and follow that. Continue until the whole intersection is outlined.