Pass use: https://www.interrail.eu/en/interrail-passes/interrail-mobile-pass

https://youtu.be/Vl0gcQPqoW4

City travel: generally NO.

Pass is not valid on Metro, Tram, Bus. In most cities it is valid on a commuter rail but often this will not be much use for a tourist. The cost of buying day passes or paying single fares within a city will nearly always be cheaper than a day of a Eurail/Interrail pass.

How and where to book seats: https://www.seat61.com/interrail-and-eurail-reservations.htm

As you list your nationality as German an Interrail pass is what you will be eligible for, if you are not from Europe then it is Eurail.

Hey, Thank you so much for answering my queris. 

Just one QQ: I have a residence permit and work at an organisation on Germany and live in Berlin. But. I am not a German National. Am I still eligible for interrail pass?

Hey, yes, you are.

Thanks

Please note that with Interrail you'll have a 2 days limitation in your country of residence (Germany in this case), no matter the duration of the pass.

If this is an issue get Eurail and travel with your non-European passport, there is no such limitation.