Everybody already has local regional tickets anyway. And most people can't be in more then one place at the time anyway. And most people stay in the same region most of the time anyway.
I live in Rostock. So if I want to go to Berlin or Hamburg (you know, where stuff like actual airports are) I am crossing "regional borders" even if it is a 200-250 km trip to each city
The point is, if you are in Hamburg, you are no longer in Rostock. So you are only using regional services in exactly one place.
Sure, long distances (I had to travel from Rostock to Tübingen last weekend) are typically not taken with regional trains (although you technically can; I did that as a poor student a few times, it just takes 16 hours instead of 10), but over medium distances (around 2–3 hours) you often have both options.
Everybody already has local regional tickets anyway. And most people can't be in more then one place at the time anyway. And most people stay in the same region most of the time anyway.
So really you are not losing much compared to having separate local region tickets in a system where the long distance trains are separated.
> who cleans and repairs the trains
The already existing organizations that have run the trains for a long time.
> who invests in infrastructure and all that
The government ...
> I always wonder how the germans can pull this off for 50 Euro. Magic.
Its not magic its just a transportation policy and taxes.