How much weight must the wall and its fixings actually support?

The whole lot.

If the cabinet weighs 20kg and you put 50kg of equipment inside it, the wall is supporting approximately 70kg. Describing the equipment as being within a 60kg cabinet rating doesn’t make the cabinet's own 20kg disappear.

And the cabinet rating tells you what the cabinet can carry. It doesn’t certify the brick, block, timber, plasterboard or whatever fixing system sits between the cabinet and the building.

Wall capacity and cabinet capacity are separate questions. Both need satisfactory answers.