The UNESCO Commission recommends, uses and promotes materials under free licenses, of course.

The ECs policy on licensing is helpful in all the talks that we had: creativecommons.org

ZDF (public service broadcaster), which also clearly explains the terms of use on its landing page https://www.zdf.de/dokumentation/terra-x/terra-x-creative-commons-cc-100.html
Federal Agency for Civic Education, e.g. https://www.bpb.de/mediathek/246349/bildungsmaterialien-bereitstellen-und-entwickeln.
The Federal Ministry of Economics provides selected graphics under CC BY, as well as all texts on the website (in principle), as noted in the imprint.
The National Action Plan (Final Report of the Federal Government) Open Government appears factually under CC BY 4.0.
The German Aerospace Center has been publishing images under CC BY since 2012

Bucerius Law School puts entire lectures and info videos online under free licenses.
The Federal Ministry of Education and Research has dedicated an entire campaign to the topic of Creative Commons and Open Access.
The OER strategy announced in the coalition agreement has just been funded and will be launched before the end of this legislative term.

For more than ten years now, the federal government and its subordinate agencies have been using Creative Commons licenses of various types, see this document from 2012
The Federal Agency for Cartography and Geodesy uses a CC BY-like license, so called "Deutschlandlizenz". It's like CC, just not international, so worse.
The BBKvirtuell series of the Federal Office of Civil Protection u. Disaster Relief is completely licensed under Creative Commons as far as I can see (CC BY 3.0)
The Federal Office for Radiation Protection has already marked videos as Creative Commons on YouTube and is currently checking to what extent other materials are eligible.
The Federal Center for Health Education decided on free licenses for the infection protection materials in May, and these are now being successively updated (in the wake of the WMF-WHO Cooperation)

Good luck!