Usually, you can also build a carport yourself without an extra kit. Just look up the construction principle online or at the hardware store and then enlarge it individually.
A building permit is generally required for carports, ergo also structural engineering approval. If you can provide that individually, of course it works. However, when making such statements, one should keep in mind that there are plenty of "DIY experts" in Germany who read such statements as above and simply copy them.
Of course, you can do that as well – but for your information, you should add that obtaining a permit afterward is always more expensive than asking beforehand.
Best regards
Dirk Grafe