and if you've a visa issued by any Schengen country it's good for travelling in all of them.
There are *some* visas issued for one country only, but it's not common, and when it does happen, they tend to be only short-term visas.
At that time a visa to one Schengen country did not necessarily mean you could travel to others.
Yes, that's how it worked originally, long term visas were (and still are) a national issue, but they quickly gave up on the idea when they realised that it was unenforceable to restrict people to one country when there were no physical borders in place.
The problem we have now with long term visas is that there's no consistency whatsoever in the application process. Some countries make it very tough to get visas, others (like Poland) couldn't care less and just issue endless amounts of them.
No. That is impossible. You would have to apply for an immigration visa in your own country. You can't do that in Poland.
Actually, it is perfectly possible. Germany allows some nationalities to do exactly that - it's listed here. gesetze-im-internet.de/aufenthv/__41.html