I have added some little own details to the box. I have inset --> then extruded some squares on the top, where weird edges appeared after the extrusion. Other weird edges appeared, on those details on the side of the box, where I have inset --> then bevelled the top and bottom edges. Then I extruded those faces too and got those strange looking effects. What did I do wrong or how do I fix this?
The first one is likely to be duplicated vertices, looks like they are bunching up the bevel. So try to select everything in edit mode and M to merge by distance, see if that helps. The second one is ok, nothing wrong with it, it's how a bevel in a corner would look. Maybe what's bothering you is that it has a triangle on top, but may go away if you add another segment in the bevel.