Fusion
Copy/Pasted Components are instances of the seed component. In essence they are pointing to the same data set. A change to any instance is in essence a change to the one data set so all instances are changed.
There is a way to create a component and use the body in the component as base geometry for another component, which then can be modified without changing the original component. If that works, however and how yo get that to work depends on how you designed your table. If you can share your design we can take a look at it and suggest a path forward.
Yes, you can simply create a sketch a extrude-cut an opening into an existing component. That sketch can be in another component. The default behavior depends on which component is activated, so pay attention to the default in "Operation" in the extrude dialogue and change it to your needs.
You can also use existing geometry and use the Modify->Combine/Cut tool to remove geometry form an existing component.
Where's the video ?
The UI for embedding videos could definitely use an improvement. Many users new and experienced run into this.
The cut extrude affects all instances at the same time.
Assuming that you want that slot on the underside of the table exactly in exactly the lengths/depth/width as sketched, you would affect different instances of that same component differently.
The components on either end would have a partial slot, and the components in the middle would have a complete slot. So that make 3 different geometries.
