I've been watching a lot of WW1/2 documentaries, and it occurred to me that one of my grandfathers was German and at a ripe age for recruitment during the second world war. He passed away when I was young, so I never got the opportunity to speak to him about it. Evidently he made a point to never speak of what he had seen or done, so my parents don't really know anything about it either.
After asking some family members though, I did discover that he had been brought in for questioning in the wake of the war, as he had been suspected of being part of the German SS, but was found to be clear of any war crimes (thank god for that).
My question: is there is any publicly available documentation that exists for interviews, interrogations etc., or any way to find his history and involvement in the war?
You should try contacting the Wehrmachtsauskunftsstelle with a private matters request:
https://www.dd-wast.de/en/tracing-request/private-matters-research.html
They are the official institution tasked with tracing and documenting the history of former German soldiers during WWII. You only need name and date of birth of your grandfather, but any additional information you can give will of course help. If there's anything to dig up in their archives, they will send it to you. However, it costs a bit of money (8€ minimum, more if you want, f.e., copies of all available documentation), and it can take a while (expect half a year, but could take longer). But even if they don't find out anything, they will usually direct you to further resources you can try. If he was a war veteran, that would be the best way to find out. If you know any other biographical information, like place of birth or residence, you should also contact the local archives, they are usually happy to help.