Same for the first two, but not the last. It's somewhat common for craftsmen to work "off the books" on smaller private contracts. It's a trade-off because obviously it's hard to claim liability when you have no proof they even worked on it and no contract specifying the work to be done. It's even more common in larger construction projects.
That's not too surprising IMHO, I would expect this to be more visible to foreigners. I myself have never seen any type of low-level corruption in my home country but I learnt that also happens quite often, it's just not too visible if you don't know where to look at.
I'm not from Germany, and do have a foreign name, I guess that may play a role.
I mean, accepting a bribe or offering one is a way to fuck someone over.
But it's not "you have to pay more". More like "just so you know, I have access to this internal tool, you pay me now and this thing that isn't supposed to be possible due to your situation is now done".
Some handymen tried to double charge me, in cash of course, but that's the only thing I've experienced. The stats say 20% of Germans report having been asked for a bribe at some point in their lives. That's not great, but not terrible either.
Source for the 20%? Anecdotally I've never heard anyone being asked for one, other corrupt behaviour though for sure, things regarding land ownership, or zoning violations.
Unreported work/income is estimated at about 10% of GDP: https://de.statista.com/statistik/daten/studie/20063/umfrage...