Terrace construction over an unheated garage

The roof certainly has a slope, that remains as is and the water flows off. Adjustable supports are placed on it and aligned, and boards or WPC are directly attached to the supports. If you are unsure, have a professional do it. If you use WPC, take solid boards and no hollow boards.

unfortunately, there is no slope on the garage ceiling

What kind of craftsmen did you have there? I mean, from which trade? In my opinion, you need roofer. The garage must be sealed according to flat roof guidelines. Insulation probably only as a "slope producer" - the garage is unheated, after all. Whether 15cm is enough, I can't say, but I would guess yes. What kind of top layer is chosen should actually be irrelevant if everything underneath is done correctly. For maintenance purposes, however, I would lean more towards one with floorboards (no matter what material), as you can remove them every 10 years or so to check the sealing and then reinstall them...

In our case, it was done once in such a way that a white screed with a slope was applied to the raw ceiling. This then served as both insulation and slope at the same time. Above that came a so-called elephant foil (I believe the material is called EPDM or something similar), which was pulled up on the sides and fixed with rails. On top of that, the support pedestals were placed and slabs laid on them. As already says, the covering is rather secondary. The slabs could also be lifted afterwards, although I generally prefer floorboards. We had this construction over a basement room and likewise as a roof terrace above my dining room; everything without any problems over many years. Is the garage roof already waterproofed?

The side facing the house was sealed with plastic. The craftsmen have differing opinions on whether insulation is necessary or not. Depending on that, we would apply either a sloped insulation or a sloped screed.

Where the craftsmen have differing opinions on whether insulation is necessary or not.

If it is a heated garage, the roof must be insulated, if it is an unheated garage, the roof does not need to be insulated but should be ventilated so that the wet car can dry.