Driving forest, dirt roads

Day was looking for something, but nothing clearly wanted. Everyone gives their mustard on this subject, but no one can prove it. Is this basically forbidden by a car on forest or field roads? (What about paths on which there are no signposts?) What happens if I do it anyway? In the “normal” forest and in the nature reserve? (fee, points, driving ban?) I hope I get some well-founded answers. Thank you in advance Jonas Buck