Yes they were shot by firing squads, that's what the mobile death squads were. The head of the SS wrote about how he even vomited at one of them because it was too gruesome for him.

And there were mass graves, here's an example:
https://www.aljazeera.com/news/2022/7/13/mass-grave-remains-of-8000-nazi-war-victims-found-in-poland