What applies to this road that is outside of a built-up area?

Your answer is correct because, outside built-up areas in Germany, it is allowed to perform a left turn under the proper conditions, such as signaling in advance and ensuring it is safe to do so.

In the context of this road, which appears to be outside a city or town (indicated by the yellow direction sign), left turns are permitted where indicated or safe. However, overtaking (passing other vehicles) and stopping during traffic jams might be subject to specific regulations depending on the type of road (e.g., highway rules vs. regular roads).

So, the key rule here is that when driving outside built-up areas, you are allowed to make a left turn if it is safe and you signal properly. This distinguishes it from areas within towns or cities, where additional restrictions might apply.

Always ensure your turn does not endanger other road users or violate road markings or signs.