Base of the refrigerator and countertop above the refrigerator

Thanks for the tip about the plug connection, I'll take a look at it.

Yes, the problem of easy assembly of the base to the refrigerator naturally remains. With my planned variant, the base would first have to be attached to the side panel and eventually pushed precisely onto the refrigerator and the other furniture. This is, of course, quite a task because the clips for the other carcass furniture then have to fit precisely so that I can simply press them in by pushing the baseboard against them.

By the way: does there have to be a gap between the carcass and the baseboard or between the baseboard and the floor, or should this be flush?

This can't possibly work. The side panel is screwed to the countertop. That only works as long as the refrigerator is not in the niche. Once the refrigerator is pushed in, the side panel is somehow attached at the bottom to the refrigerator to ensure it stands securely. Alternatively, you could seal the side panel to the floor with silicone. That would adequately secure the side panel against lateral movement from one side panel. The toe kick (in Ikea language just "Sockel") runs along the entire row. It should be enough if it is only clipped onto the cabinets. If not, it can be secured to the side panel using the aforementioned clips, for example. Before installing the side panel, I would check whether it needs to be shortened in width in case the refrigerator door does not open wide enough due to the protruding side panel. Whether the toe kick aligns with the floor or the carcass is irrelevant. The only important thing is the cutout shown in the assembly instructions under the refrigerator for ventilation.