Not a lawyer but the German constitution, Article 12a, speaks of men above 18, not of citizens, or even residents of Germany.

https://www.gesetze-im-internet.de/englisch_gg/englisch_gg.h...

So that article can in theory be used to conscript any man, citizen or not, living in Germany or not.

The Wehrpflichtgesetz, which is a simple law and requires just the 50% Bundestag majority to have it changed, refines this very wide constitutional power in article 1, to require men who hold German citizenship above 18.

https://www.gesetze-im-internet.de/wehrpflg/BJNR006510956.ht...

Article 3 refines it even further to folks below 45 or 60, depending on the severity of the situation.

But yes, in theory it can be changed to include any non-German citizen man, people aged 80, living inside of Germany since a while or never having been to Germany ever, or just random men who happen to change flights at FRA.