Which countries are covered under long-term work permit “Deutschland” visa?

By covered, if you mean the right to work, then it's only Germany. The work permit of a Schengen country does not give you the right to work in any other Schengen country, only in the specified one.

On the other hand, if you have a residence permit in a Schengen country, like Germany in your case, this gives you the right to move freely in the Schengen zone, as long as you stay less than 90 days in any 180-day period in the Schengen zone outside your permit-issuing country. Obviously you can spend the whole year as long as your permit allows in the country from which you got the residence permit.

Check out the list of Schengen countries here. Notably, the Schengen area does not include the EU countries UK and Ireland, but it does include several non-EU countries, such as Iceland, Norway and Switzerland.

On top of that, there are several countries not in the Schengen zone, but they allow visa-free travels if you hold a Schengen residence permit. For these you should always check the most recent regulations of the country that you want to visit, but some examples as of the writing of this post are Mexico, Bulgaria, Romania etc.

Also note that, although considered sovereign Danish territories, a Schengen residence permit does not give you visa-free travel rights to the Faroe Islands or Greenland. Similarly some restrictions apply to the overseas islands of France and the Netherlands.