Complex sentence: However, two of the mentioned death camps (Auschwitz and Chełmno) were situated in lands annexed by Germany (Germany proper by Germany's own account) and also, most Nazi concentration camps were located in the territory of Nazi Germany anyway.
Split into simpler sentences: However, two of the mentioned death camps (Auschwitz and Chełmno) were situated in lands annexed by Germany (which the Nazis considered to be Germany proper). Most Nazi concentration camps were located in the territory of Nazi Germany anyway.