How to insulate the outside of the garage from water?
We are talking about a garage that borders two sides directly on the ground because it is on a slope. Thus, the back of the garage must support the slope. These two walls and the ceiling of the garage are cast in concrete and reinforced with steel. All the others, which are freestanding, are brick.
Should the walls to the ground need to be further insulated?
Yes, you should. The areas in contact with the ground should be blocked off. Usually 2x thickly coated, then honeycomb membrane, possibly drainage (required for pressurized water).
I would also do a 2x thick pavement (you can add perimeter insulation if you want, but it won't do anything for the garage) and then put drainage channels with drainage underneath instead of the pit membrane.