Do 90% of all German Chemistry graduates start a PhD? Can someone corroborate the fact above? If it’s true, how did it come to such a high number, do they have some slots reserved, or is it easy to enter German PhD programs? Would it also be easy for foreign graduates (in my case chemistry) to join a German PhD program?
Source 1 (German Wikipedia):

Rund 91 % der frisch diplomierten Chemiker begannen nach ihrem Abschluss mit der Promotion.
91 % of the recent chemistry graduates enrolled in a PhD program. [This does not imply they completed it.]

Source 2 (Universität Duisburg–Essen):

Wie jährlich durchgeführte Erhebungen der Gesellschaft Deutscher Chemiker (GDCh) zeigen, begannen in den letzten Jahren über 80 % der Absolventen nach dem Diplom bzw. Master mit einer Promotion.
According to yearly inquiries by the Gesellschaft Deutscher Chemiker (GDCh) [Society of German chemists], 80 % of diploma or master graduates enrolled in a PhD program in the last years.
According to the German Chemical Society, apparently 90% of master's recipients in chemistry do start doctoral studies afterwards.

Partly this is because PhD "admission" is largely not an admissions process at all in Germany. Individual faculty members who receive grants can hire master's recipients as "Wissenschaftliche Mitarbeiter" (literally "scientific worker," but "research assistant" is a better translation). They can do this without recourse to going through a department-level admissions process, and can skip outside advertising altogether if they have an internal candidate they can appoint instead. And, since many students follow the rule of "Was der Bauer nicht kennt, isst er nicht"—"What the farmer doesn't know, he doesn't eat"), many students do their entire education at one school.

So yes, it's much easier to get a PhD position if you're already in Germany.