I know that I have to provide detailed information which data I save
and how I use it.
Even if you don't access files and information - such as personal information or files - you still need a clear TOS for your App, for both your users' and your own legal protection.
But I am a poor student who don't want invest money into a lawyer to
Being a free or paid App doesn't make a difference when it comes to the TOS, if you need one, and if you need to pay for one to be drafted.
But not investing money in a lawyer can be a critical mistake if your App will be distributed on the popular App directories like Google Play and iTunes and your TOS does not clearly and legally outline all possible usage and data polices. Lawyers have experience in covering all the bases.
Sure, you can: https://www.google.com/search?q=privacy+policy+generator But you run considerable legal liability not having a TOS that correctly and legally addresses all users' privacy concerns and clearly outlines what you do to insure security. That's what lawyers are for.
Is it possible to just create some “I save all your data“ policy which
would of course be to much but it would include all data I save and so
I would not miss anything in my policy?
Sure, you can simply say I save all your data. But how can you be sure that you've haven't missed important policy details and scenarios that could result in legal action against you? Like how long do you save data? When will you delete it? How secure is it? Can the user ask for deletion? What happens if you get hacked? What about users in countries other than the US? Users under 18 years old? You must use arbitration for any disputes? And on and on.
The considerable legal liability of using a non-professionally drafted TOS means you run the risk of getting sued. You have to decide if it is worth the risk.