I will be passing by Amsterdam to visit friends via Frankfurt airport, will that be eligible?
The Schengen Area counts as a single country for border purposes (hence the common visa), meaning that, unless you hold an LTV visa (which is an exceptional thing), you're free to move within the entire area while the visa is valid.
So yes, you can go to Amsterdam.
And a single entry means entering only one country in Schengen, or only one entry to Schengen and if I exit Schengen I can't reenter.
It means you can only enter the Schengen Area once - once you exit you need a new visa to re-enter
When going from Oslo to Frankfurt, you'll depart from the international section, but without passport control, because Norway is in Schengen. The "single country" is Schengen, not the EU