Which networks you actually get, and where that shows
German plans use O2 and Vodafone. Deutsche Telekom, which has the country's strongest rural coverage, is not in the roaming set.
For most trips this is irrelevant. Berlin, Munich, Hamburg, Frankfurt and Cologne all have solid 5G on Vodafone, the Autobahn corridors are well covered, and the ICE routes between major cities are fine apart from tunnels.
It becomes relevant if your itinerary is rural. O2 in particular has known gaps in rural Bavaria and the Black Forest, which are exactly the regions people visit for hiking and driving holidays. If your trip is Alpine foothills, forest villages and back roads rather than cities, plan for stretches with no usable data and download offline maps in advance.