Insulating garage roof, correct sequence of execution

No, it is not a must - but certainly not the dumbest idea. Also not expensive...

Thank you for the information, we will add this to the list of "points to be clarified."

Is it generally always necessary to insulate the garage (if directly attached to the house including the passage to the utility room)?

Whether insulation or not, sufficient ventilation is IMHO important (at least if the car is driven in and out daily). In addition, I would insulate the roof (!) to keep the greatest heat outside in the summer. A good alternative is green roofing.

I put a warm car in - every day again the same and suddenly it’s warm - for hours

I can’t imagine that a warm car has so much thermal energy that it significantly heats a well-insulated but well-ventilated garage when it’s really cold. The heat capacity of the walls/floor alone is probably so high that not much happens there, especially if it only occurs once a day. From on the road

I can't do anything about your lack of imagination - well, you won't achieve much with a small 4-cylinder diesel anyway.

By the way, I wouldn't ventilate it in winter as long as there is no wet car inside.

Unfortunately, at least here it is the case that it also involves cold wet weather. For me, the effort would be too great, sometimes ventilating, sometimes not... Should one then open/close the ventilation openings depending on the weather? That would be too much work for me to change 100 times a year. I don't even drive a diesel, but if you don't have stop-and-go city traffic right in front of the house, a car in winter doesn't really get hot. Apart from a few engine parts, there's not much warm mass in there. The best test is to sit in the car without the engine/heater for a few minutes in winter to see how quickly it cools down.

from the road Edit. A four-cylinder is nowadays no longer a small engine at all, small are the three-cylinders, which already have over 100 hp, which is more than suitable for everyday use.