You’re supposed to report* at a local clinic/hospital or via the EUCARE app. See my recent experience here and the blog post @lostinasia linked to:
Yeah, it’s surprisingly difficult to find government guidelines for this, at least in English. I couldn’t find anything either.
7 days, which is really 8 days, or maybe longer. 
See the hassle I had.
Still 7/8 days or longer as above. A negative test doesn’t affect that, and it’s not a requirement for leaving quarantine either from what I understand.
(*My phrasing here is deliberate.)