They have only 1 foreign player (Austrian) and neither one of their German players was ever capped for the German national team. Few of them had occasional U-19 or U-20 appearances.
Heidenheim is super small club from a 50k town in the middle between Stuttgart, Nürnberg, Munich/Augsburg. Just 15 years ago they were still playing in 6th division and worked their way up from there; promoting to 2. Bundesliga only some years ago. They are a low-profile club doing solid work and having a very good coach. They live by a strong team chemistry and always play very disciplined. None of their players have the quality to play in Bundesliga, let alone in german national team. Some years ago their captain Schnatterer had the quality to play Bundesliga, but he decided to stay in Heidenheim and is a legend there. And I doubt he would have worked in Bundesliga, he is just a Heidenheim guy and would fail at any other team imo, because he wouldn't feel comfortable there.
Having a player from 2. Bundesliga playing in german NT is a very rare event. The last one was Jonas Hector iirc, who stayed at Cologne, when they relegated, but I can't remember another player.
I think it won't be easy for Werder today, because Heidenheim can be a pain the ass to play against. You still have this horrible home record from Werder, but they should be on a high after last weekends miracle. Heidenheim is poor in away games and I expect them to just sit back today and being happy with a 0-0 (or any scoring draw as well). In the end Werder should win this today all things considered, but I can't imagine it being a big win, something like 1-0/2-0/2-1 seems likely.